Imagine nitrogen is like tape, and it's easy to stick it onto something.

Normally, when you apply tape to something, you can do a little bit of work to remove the tape. But imagine two pieces of tape stuck to each other-- Nearly impossible to separate, at least without a lot of very careful work.

The particular form of nitrogen in the air, N2, is two nitrogen atoms stuck together, and like the tape, and they're very hard to separate.

Many/most plants rely on nitrogen fixation, which is a collection of processes for separating the two nitrogen atoms and sticking them to something else that's easier to for the plant to work with/consume.