I think Miles and Tigress pretty much got it. It's not a phrase I use, either -- I don't talk in bumper stickers -- but "let God" is a principle I sorely need to learn.
I love to be in control. I love to make things happen. If someone says "We should go see a movie!" and several people agree, I get into the details and make it happen, otherwise ninety-five times out of ten it ain't gonna happen.
So I'm kind of the same way with God. If it seems like things just aren't moving along fast enough,I'm the farmer out there at night gently pulling on the plants, making them "grow."
So, to me, "let go and let God" means that I should do my part (sowing and watering) and trust God with the outcome,even if it's an outcome I don't like. I generally only trust him with the outcome if it's what I had in mind, and if things aren't going according to my plan, I start doing things to make me feel better -- spending money I shouldn't, eating more than I should or eating out when I shouldn't, treating myself to entertainment when I should be doing chores or studying, etc. I decide that God's not doing enough to satisfy what I consider my quality of life needs, and I take things into my own hands.
I'm learning about how to take comfort in the Lord rather than, say, sushi and a movie, but it's not easy for me. "Comfort in the Lord" seems to be about trust and patience, I think, and those things are hard for me.