Simply put a 4:3 picture is 4 units wide for every 3 wide. it looks like a rectangle. SD (standard definition) televisions had screens shaped like this
Sometimes a 4:3 picture is referred to as "Academy Ratio" for the Academy of Motion Picture Arts and Sciences. This aspect ratio dates to the early days of motion pictures when the visible area used in 35mm film used in cameras was in a 4:3 ratio. Sound films changed the ratio a bit to 1.375:1 but it's close enough. All films shot in 35mm between 1932 and 1952 used that ratio.
When television was created, it used the 1:33:1 ratio. Eventually the American TV broadcast standard had a name, NTSC. You could put a 1.375:1 film on an NTSC TV with minimal loss of picture. That scared the big movie studios, they wanted people to come to theaters not sit at home and watch TV. So they came up with the idea of "an experience at the theater you couldn't get at home". There were several ways of doing that.
One was increasing the use of color film. Even after the great technicolor films of the 30's, many films that probably should have been shot in color, weren't. While NTSC's color system was created in 1953, color sets didn't become common and affordable till the mid 60's, so color was an advantage the movies had over TV.
Another way was improved sound. While there had been previous experiments in multi-channel sound, it really started to hit it big in the early 50's. TV didn't have Stereophonic Sound till public TV station WTTW Chicago sought a way to improve the sound of it's music television show Soundstage without using kludgy simulcasting on a friendly FM radio station. It helped invent the MTS Stereo sound system for use with NTSC that was implemented in 1984
And then there was the widescreen picture, called by various names like Cinemascope, or Panavision, it was designed to make movies an experience. A widescreen picture looks like something like this 16:9 image
Glorious Technicolor, Breathtaking Cinemascope and Stereophonic Sound
And it worked! Ben Hur! Seven Brides for Seven Brothers! North by Northwest! These were experiences you couldn't get at home. If you wanted to show a widescreen movie on a 4:3 TV, you either had to "letterbox" the picture, or pan and scan.
Letterboxing looks like that video above. The picture is fit to the width of the screen leaving black bars at top and bottom. You get all of what the director wanted on the movie screen but get a smaller picture on your TV. Some people were even upset about those black bars, thinking that they paid for all the picture, not black bars, because they didn't understand aspect ratios.
Pan and scan is where an after-the-fact editor simply decides which part of a widescreen picture is used for a 4:3 image on TV, and cuts out anything not shown. So with Fred dancing there, they might decide to focuson Fred Astaire and then you might not see piano, or Janis Paige if she's far away from Fred. Putting it bluntly, Pan-and-scan is evil. Turner Classic Movies did a short informative video about Letterboxing and pan and scan. The piece was done before TCM began broadcasting in widescreen so the video itself is 4:3!
When studios began releasing a few letterboxed versions of films on VHS back in the old days, The FHG was much pleased. The FHG was even more pleased when widescreen releases became more common with DVD.
Alongside DVD's came the first EDTV's and HDTV's. EDTV's are "Enhanced Definition Televisions", better than SD, but not quite HD. Usually they were 640x480 screens progressive. They were the first sets to give us better than SD quality video. They were also expensive and still 4:3.
Now the people in charge of the TV standard had been discussing a new TV format for years, but hadn't been able to agree on anything. They almost gave us a 720x480p analog format back in the 80's-90's. One of the big impeti behind upgrading TV technology were the new playtoys we were attaching to TV's, video games, video players, even computers. Those things needed more resolution and detail...and eventually they got it in 1998.
That it was the ATSC television standard, and now manufacturers could actually start making HDTV's to match the standard. ATSC defines several types of picture, but the highest resolution is 1920x1080 progressive, usually called 1080p. 1080p, as you can guess, is a widescreen ratio. While some of the super-widescreen 1.87:1 and 2.39:1 formats used in movies still have slight letterboxing on 16:9 screens, the era of Pan-and-scan is gone. Good riddance!
But now we have a different problem. All the old 4:3 content. Old movies up till 1952, almost all TV till the early 2000's, and most (but not all) video games up till 2006 is 4:3. When you display 4:3 content on a 16:9 screen you get pillarboxing. Black bars on each side of the picture, like this:
That is how a 4:3 (or standard definition) picture on a widescreen TV is supposed to look. If the picture is stretched to fill the screen, your system hasn't been set up properly which makes The FHG cry. Whatever you do, DON'T stretch the picture. Media should always be displayed in it's native aspect ratio and preferably at it's native resolution. The FHG does this and you should too. More on HDTV's in another post.