They include beans, peas, lentils, etc.
They are excellent source of plant protein (incomplete, but combined with other plant proteins becomes complete).
They are also excellent source of complex carbohydrates, which makes them a great source of steady, reliable energy.
They also have lots of vitamins and minerals.
Problems can include: causing slow digestion, gases, blocking absorption of some other nutrients.
They need to be soaked overnight before cooking.
They also need to be cooked thoroughly, you can’t eat them raw. (especially beans need to be cooked for a long time, while fresh peas don’t need such a long cooking)