I just put mine on the warframe I use the most.
The lens takes a small percentage of the affinity that you would have earned and gives it to you as focus points. It only works on level 30 equipment, and if you forma the equipment you have to get it back up to 30 before it gives you focus points again.
You use these focus points to unlock perks related to your focus school. For example, my school has a passive that regenerates energy throughout the mission. These passives only work once you turn them on in-mission by using your focus ability (hold down 5 by default, assuming you're on PC).
Once you have a lens equipped, you're also going to see yellow orbs start showing up in missions. If you pick them up, they give you extra focus points for a bit.