It can be used for anything from learning the alphabet to revising irregular verbs. There are many variations dependent only on the teacher's imagination.
The basic idea is that the teacher covers the whiteboard randomly with words or letters. If, for example, you are teaching the alphabet, you might cover the board haphazardly with all the vowels and some of the more problematical consonants. It's a good idea to repeat some of the letters, perhaps in upper and lower case.