Most legal terminology is hard for laypeople to understand.

Literal

Legal terminology [possessive-の] majority [topic-は] laypeople [dat-に-は] hard-to-understand

Note the double には pattern marking the experiencer — 'for laypeople, it's hard to understand'. わかりにくい is a compound of わかる + ~にくい ('hard to do'). 素人 (しろうと) = 'amateur, layperson'; its antonym is 玄人 (くろうと, expert/pro). Both words are used heavily.