Hi....I am pretty sure that this topic has been covered before, so do a search and see what you come up with. Maybe one of the moderators can post a few links for you to browse.
In a nutshell, decelling means that you take a lighter course load and in effect spend an extra term on the island. Most decels occur in first term and are the result of either Gross Anatomy or Biochemistry. Decels do occur in higher terms but to a lesser extent.
The natural progression of a decel is as follows: after the midterm exams in first term, a student is failing one of the aforementioned courses. The dean of students will then advise the student to enter the decel program, which means that the class being failed is dropped and the student continues with the other classes of that term. An alternate schedule is then followed whereby the student will take fewer credits (usually one class less) per tem until they leave the island. I did not have the "misfortune" of decelling so I am not 100% sure about the details. Someone correct me if I'm wrong on any of this.
One thing to add is this: you can choose to ignore the dean of students' advice to decel and attempt to turn things around and pass the class by finals. However, if you do end up failing the class, you'll be up the creek and facing an "I told you so" situation....
Hope this helps a bit,
p.s. note I use misfortune loosely since I think that decelling is not necessarily a bad thing. It can make a world of difference. I know people who were struggling before decel and then proceeded to make straight A's after decelling (one person is particular later transferred out to an American school despite decelling).