It is possible to use Skype with voice only - you just do an audio call without video. But sometimes teachers simply forget to write on their profile that they have also video. If you are interested in a teacher you should ask that teacher. Personally I find that video is important because without it it's hard to know whether the other person understands you or not. The expression of the face is very important, and it helps communication better than words, sometimes. So I much prefer to use video, although some students would rather have a session without, and that can work too.

That means they don't have cameras.