A Module is an equip-able item that arrived in version 4.6.
There are currently 8 passive modules, and 5 active modules.
This type of module is freely available to all pilots. They add an activation button to the battle interface, and are activated in a similar manner as robot abilities. However, in order to use (activate) their abilities, a currency called Power Cells are required. Power Cells can be purchased for gold on the robot load-out screen, or by progressing through the Battle Rewards system.
Currently, all active modules (Except for the Advanced Repair Unit, which costs 40) require 20 Power Cells per activation. Not all robots come with an active module slot, however, no robot has more than one slot. Lastly, unlike passive modules, these cannot be upgraded.
This type of module is purchasable via the store, and are available for gold or silver. They can either give you a permanent stat bonus, or a one time special ability activation (that activates in special circumstances).
All robots have at least one passive module slot, up to a maximum of 3. The first passive slot is unlocked on a robot once it reaches level 3, the second (if available) unlocks at level 6, and the third (if available) unlocks at level 9.
Lastly, like equipment and robots, they can be upgraded with silver, up to a maximum level of 4 (for cheaper modules) or 6 (for more expensive modules)
Note: Unlike equipment and robots, passive modules cannot be upgraded to Mark II.