The Law is simple but very hard to do. Love God, love your neighbor. Simple as can be, but terribly hard and difficult and we rebel against it.
Legalism tries to reverse this. It thinks that by complicating the Law, you can make it easier to do. If you define and get exact and this and that, you can just follow the "divine" checklist and that is it... and in reality legalism becomes nothing but a pious excuse to avoid love that is hard.