Extensive reading (what you would think of as leisure reading) differs from intensive reading (what you often do in class). The teacher does not hand out vocabulary lists, you are not supposed to look up words in a dictionary. No one is going to go over grammatical points with you. You will not be tested on your ability to read aloud fluently.
Extensive readings follows the philosophy that readers should choose simple books and read lots of them. The act of "extensive" reading will build up fluency and a rich vocabulary without causing you to get bogged down in minutia. There are 4 golden rules to follow
The challenge to the teacher and librarian is to help the readers find enough books at the right level so that there is plenty of fuel to feed the fire of reading. If you want to read more in Japanese, please please don't hesitate to ask for reading lists.