This is nothring more than declaring an array of ints, called diffWays. And it will be able to hold one more element than whatever "size" is. In the code you posted, it would be able to hold 201 elements.
The best way to look at this is (like always) to break it down...
This means "Taks whatever is in diffWays[j] and add something to it, then put that back in diffWays[j].
What are we adding to it?
so we need to figure out what "j - availableCoins[i]" is. j is an int, avalableCoins[i] is simply some other array element.
Never be afraid to put println statements in the code an run it again. Watch what happens to each variable at each step. If something is still confusing, put in more print statements and try again.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors