Many items that a captain can fit (like weapons, cargo expanders, etc), have an energy requirement. To fulfill this need for power, a captain can fit reactors on their ship. Please note that you will need to invest IP into the Reactor Tech skill to fit reactors.
A full list of reactors is available at the reactors category.
You can not fit more than 1 of the exact same reactor (model) onto a ship. You can fit reactors of different types (or levels, to include crystal modified +P1, +P2, +P3, RA-xxx, etc) onto the same ship.