Lapsed spending occurs when a department does not spend all of its allocated spending, the spending that Parliament has authorized it to spend. It can occur for a variety of reasons. It can be due to delayed proposals or a delay in acquisition.
Off the top of my head, usually the departments that have the biggest lapses are also those with the biggest budgets. The Department of National Defence usually comes at the top of the list, because it has major procurement projects that can be delayed for a variety of reasons.
I don't remember off the top of my head who is number two, but usually the bigger the department, the more likely they are to be in the top five departments that lapse.