Rereading seems to be the favorite learning method for a lot of my college associates. At least as I was talking with them about our learning habits.
Rereading simply means reading a text more than once. It can mean reading the whole text again or just parts of it.
But rereading by itself (talking here about mass reading the whole text) in not really effective use of time.
First of all, rereading the whole thing, without having a specific goal will make a reader much less excited about it (even rereaders of mysteries have reasons for rereading - fron fun, seeing how all the clues came together,...). But for studying doesn't count. There is almost no motivation in it.
It does count, if the reason is like misunderstanding the concept or looking for applications of the concepts.
Second of all, rereading the same text immediately can actually harm on the long run (exception is, not understanding on the first reading). Since the second reading brings the feeling, that material is easier or better remembered than it really is. Making the learner to overconfident in his knowledge.
That happens, because the material is still fresh in the brain, meaning the retrieval of it cames from the short term memory. But having a material in the short term memory, does not equal long term understanding (but gives us the same feeling).
If the text was not understood the first time, the rereading might help. Also, if there is a concept, that need better understanding, the rereading might also helped.
But using the spacing effect, rereading is helpful, since longer intervals bring less overconfidence. The reason is, that the text is no longer in short term memory, but has to be retrieved from a long term memory.
So to try rereading, read a text.
Reread any parts you didn't understand.
Then schedule rereading if that text for some time in the future - longer than 1 week at least.
You can read more on the spacing effect here.