SDTV stands for Standard Definition Television. It is a type of television that shows images and videos in a lower quality than the newer types of television like HDTVs or 4K TVs.
So, imagine you are watching your favorite cartoon on your mom and dad's old TV. That TV is most likely an SDTV. The images might not be as clear and bright as they would be on a newer TV, but you can still see and enjoy the show.
SDTVs usually have a less sharp picture and less vivid colors than HDTVs or 4K TVs because they have fewer pixels. Pixels are like tiny little dots that make up the picture on your TV. If your TV has more pixels, the picture will be clearer and more detailed.
So, although SDTVs might not have the best picture quality, they can still be a great way to watch your favorite shows and movies if you don't mind the lower quality.