Okay, imagine you have a bunch of toy blocks with different colors and shapes. You want to stack them up into a tower but you can only put a bigger block on top of a smaller block.
A "permutation" is like a list of numbers that tells you the order to stack the blocks in. For example, 2 1 3 means you stack the second block on top of the first block, then stack the third block on top of the second block.
A permutation is "stack-sortable" if you can use the rules of stacking to make the blocks form a tower according to the order in the permutation.
So, if you have a stack-sortable permutation, you could take your toy blocks and stack them up exactly like the permutation says. But if you have a permutation that isn't stack-sortable, you wouldn't be able to stack them up in that order because some of the blocks wouldn't fit on top of others!