It's when you leave all your studying to the last day or so and then try to "cram" all the information into your head before the exam. Normally you're supposed to study bit-by-bit everyday and learn it normally instead of trying to learn too much in too little time.
Okay that sounds like what I did before my exams:D, Omg I am a crammer, I feel I remember more if I read everything the day before the exam.