Shifts are just a concise way of measuring how effective your particular attempt at something is, or a particular effect is.
On a dice roll, it's the difference between the difficulty and what you rolled. So if you need a Fair (+2) to succeed and you get a Great (+4) roll, you succeed and have two shifts.
For some actions, it's important to have shifts (like pretty much everything in a conflict), because they govern how much damage you do with an attack, or how far you can move, etc. It just depends contextually on the action. For some actions, it doesn't matter as much, or having shifts lets you do something quicker than usual, or some other special effect.
For stuff like magic, you just talk about shifts directly without making a roll, and they just have a certain effect depending on what kind of action you're performing. So, like, when you summon power for an evocation attack, you just state it as a number of shifts. Those shifts translate directly to damage.
So I want five shifts of power for my attack, right? That means my spell will do at least 5 stress damage no matter what I roll, provided I hit my target.
Does that make sense? Do you have a specific question I can answer that might help more?
-L