Modules are any item that can be fitted into a ship which will allow the ship to perform certain actions or boost its performance. Examples of modules include weapon systems like Missile Launchers, sensor equipments like Signal Amplifiers, and repair items such as Armor Repair Systems.
All modules are added to a ship using the Fitting screen. All ships have a specific number of slots into which modules can be placed. These are divided into High Slots, Mid Slots, and Low Slots. Each module can fit into only one type of slot. For example, weapon system only fit into High Slots.
Some modules also require a certain amount of CPU and Powergrid to be assigned to them to be active. If you try to slot a module without the required CPU or Powergrid, the module will not be active in the ship.