Everything you look at on the internet is downloaded to your PC before you can look at it.
For text this is no problem as it is just plain text.
For images they are usually in jpg and gif format so are compressed, but it is "worse" than text and if you download a lot of images it does add up.
Video is often buffered, so it only has to download a part of the video before it starts playing.
If you only watch part of a video then probably only the part you have watched has been downloaded (5Mb, 10Mb etc)
.
But if you watch all of a 20Mb video then all of it is downloaded and this will count as 20Mb towards your 5Gb limit.
But remember that 5Gb is 5,000 (5 thousand) megabytes
so you would have to watch a lot of 20Mb videos for it to worry you.
It is mainly for people who download whole movies from the internet which can be 500Mb or more per movie.