Video is sent at several qualities at once and your device picks whichever your connection can keep up with. That is why a video can start soft and sharpen after a few seconds — it is working as intended, not a fault.
It keeps stopping to buffer
In order of how often it helps: try another network (mobile data instead of wifi, or the other way round); close other things that are downloading; and if you are on wifi, move closer to the router. If one video stutters and everything else is fine, tell the site owner which one — that is useful information and they cannot see it from their end.
Subtitles
If a video has them, there is a subtitles button in the player. Not every video has them; they have to be added to each one.
It will not play at all
Refresh the page first. If it still does not play, try a different browser — that tells you and the site owner whether the problem is the video or the device.