A manual for that thing (I am guessing that it's the TB100xx9) is mighty elusive. I found a few hints, though.

"Full screen" mode and a standard 4:3 TV will result in 16:9 content being stretched (distorted). And when you get 16:9 content that includes pillarboxes, it will be stretched and have black bars on the sides. Unless you are using the box with a 16:9 TV, I'd recommend avoiding "full screen" mode entirely.

"Zoom" will give you the familiar viewing mode for 4:3 material (no bars on the sides or bottoms), but it will lop off the sides of 16:9 programming.

"Letterbox" gets you the original aspect ratio for widescreen content, but you may run into some instances where you get a full frame of black bars. If the box knows it has a 4:3 TV and the station is broadcasting 4:3 content without doing it pillarboxed as 16:9, those instances may be limited to commercials only (which is not much of a big deal). You'll get TV shows with black bars above and below in many cases, but we've been getting that on a lot of shows for years now.