A particularly bad form of leak is one that is rectangular in shape - and the bigger they are, the worse they are.
They are formed when you play two long edges (long meaning 3 squares or more) at right angles to each other.
Take a look at the Yellow N here.
This creates an L4 leak, which is worth thinking of as a (half) rectangle. An opponent's L4 piece (or possibly a bigger one, such as P or U) can fit there perfectly and we cannot stop it!
We should always avoid rectangles if possible because they create big spaces where we can't play but our opponents can, they limit our corners, and they keep our corners far apart (limiting flexibility).
Rectangles are also bad because they almost always lead to flexible leaks.
That means "a space where many different pieces will fit".
In this case, Yellow could use a different orientation of the N5 to create a much more favourable v3 leak instead, as seen here.