Oh, thanks, I'll try to make the description there clearer, it's a confusing little game.
I'll try to rephrase it here: you have to protect the green circles, and at the same time you have to try NOT TO protect the brown circles. If you protect them all, you can make the game go on indefinitely, but that's not necessarily a good thing, as brown circles hurt your score for as long as they're there. If a brown circle gets shot, it turns green. If a green gets shot, it's gone.
The power measure is those eight yellow lights above the 8x8 multicolored grid (I guess that would've been obvious in a real Meggy Jr., but not here). But remember that you can't place a shield overlapping another one, no matter how much power you got!
You are supposed to end up losing all green circles at some point, though, at least in theory. You should just try to keep them going as long as you can.