Forma allows you assign a polarity to a slot in a warframe or a weapon. A ‘polarity’ is just a ‘type’, each mod falls into a catagory, and so forma-ing one of your equipment slots you are matching the type on your mod to the type on your equipment.
Doing so gives you a discount of 50% on the amount it costs to equip the mod.
Using Forma on Equipment
To apply a Forma, click Arsenal and equip the desired item. Scroll over the item in the arsenal. Select Action at the bottom of the screen, then choose Polarize, which will show a layout of the item’s mod slots and polarities. Click on a slot to cycle through the possible polarities. A slot that already has a polarity (either by default or from a prior polarization) can also be changed.
Forma can only be used on equipment that has already been ranked to 30 and, when used, said equipment is reset to rank 0 (unranked). Warframes and Warframe Skills will be reduced to rank 0. Forma can be used again on already polarized equipment, provided that it is leveled to 30 again. Orokin Catalysts and Orokin Reactors are unaffected by the usage of Forma. Re-leveling equipment after using Forma on it does not grant mastery points.
For each Forma used on equipment, said equipment will gain a star above the name of the equipment starting from the center.