Forma is an item that can be used to modify and manipulate polarities of warframes or weapons in the game, making for more efficient mod-setups.
It’s quite easy to Forma a weapon or a warframe you simply go to your arsenal and select the weapon or warframe you would like to Forma (gotta have crafted a Forma for this first), the you go into the bottom-right corner and choose “actions” then “Polarization”.
This will bring up your modslots where you can click on the one you wish to alter polarity for, for each click you do, the polarity scrolls through list of possible polarities to choose from.
Important note to make when doing this is that after you Apply the polarity change you want, the weapon or warframe will reset to level 0 and leveling it up to 30 again WILL NOT give mastery rank. You only get this one time for weapon/warframe.
A very useful reason to Polarize a warframe for example could be to alter/change your Aura slot Polarity, so that you can have for example – Polarity, instead of V polarity – so that Corrosive Projection and Energy Siphons can be used effectively instead of something less useful :)
Another Important Note to make is that its a very Tedious process to Forma something, and you gotta make sure you are 100% confident in your choice of Polarity because it will affect all the future builds you will make for that weapon or warframe, and redoing it is just a waste of time as well as Forma.
There is however the option of “Swapping polarities” in Action tab for weapon/warframe which could help out if you are to place a polarity in the wrong slot so you don’t have to redo the entire Forma process :P