“…Increases the durability of all shields and increases the regeneration speed of energy shields. Effect stacks with other shield bonuses…” – in-game description
The Fortifier is a passive module.
This module grants a stackable % bonus to the durability of either built-in or mountable physical and energy shields (except Absorber), as well as giving energy shields an increased regeneration rate.
This unique effect the passive module grants you is especially effective with energy shields, as it grants them a double-bonus. An Ancile on a robot using Fortifiers not only gains extra durability, its regeneration rate also increases, making it harder for robots sporting Punishers (or Atomizers, if you're using an Aegis shield) to attack you directly as the energy shield might even be able to keep up with their firerate.
If you own an Ancilot (Lancelot+Ancile+Weapons of choice) or a Bulwark, Fortifiers are technically essential. Lancelot's physical shields gain much-needed increased strength, and its Ancile also gains the double-boost the Fortifier provides energy shields with, technically giving Ancilot a triple-boost using only one Fortifier. Bulwark, on the other hand, is even better, as it gains the double boost for an Aegis barrier, at the basically unnoticeable cost of uniting its physical shields into one. Also, compared to Ancilot, Bulwark gains more firepower.
Being a passive module, you cannot equip the Fortifier onto a Titan, as they can only use their own specific kind of module, the Titan Modules.