Bleeding them means to get the air bubbles out. Usually, you don't have to do this, however, if you do end up removing a brake cylinder (disconnecting it from the brake line), You get air in the brake lines.
It is real easy to bleed them. If you look on the inside of the tire, you'll see a little spout near where the brake line connects to the braking system at the tires (looks sort of like a grease fitting spout).
It takes 2 people to bleed the break system. One to pump the break pedal, one to loosen up the spout and allow the air bubbles to squirt out.
Make sure your brake fluid resivoir is full first, then have someone to pump the brake (I usually do this with the engine off, but you can do it with it on). Tell them to pump the brake 4 or 5 times (building up pressure), then hold it down. While they're holding it down, loosen the spout, and watch out for the spray, only loosen it a little. If there is air in the lines, you'll see it "spitting" sort of. Then a steady stream should come. When it is steady, tighten the spout again. You might have repeat the process a few times to get ALL the air out.
It is really easy. Usually you only have to do it to the affected brake line (there are main 4 brake lines--one for each tire), but if the brakes still feel funny, do move to another tire and so on. Should take a grand total of about 8 minutes or so.