The word nut has several meanings:

1. A hard-shelled seed or fruit that grows on trees and contains an edible kernel inside. Walnuts, almonds, pecans, and cashews are all nuts. You have to crack open their tough outer shells to reach the tasty part inside. Squirrels famously gather and store nuts for winter, and people have eaten nuts for thousands of years because they're packed with protein and healthy fats.
2. A small metal piece with a threaded hole in the center that screws onto a bolt to hold things together. When you tighten a nut onto a bolt, they lock together and fasten two pieces of material. You might use a wrench to tighten the nuts on your bicycle wheels or see mechanics using nuts and bolts to assemble machinery.
3. Informally, an enthusiastic fan or someone obsessed with something. A baseball nut knows every player's statistics and never misses a game. A computer nut builds their own machines and reads about new technology constantly. The word suggests affectionate intensity: being a nut about something means you're passionate about it, maybe even a little over the top.

Less commonly, nut can mean someone acting foolishly or a tough problem to solve (a tough nut to crack).