If you have air in your brake lines, the brake pedal will feel "squishy" and will not give you maximum braking power. You will probably find that pumping the pedal temporarily alleviates the problem.
All that said, let's visit the issue of *why* you might have air in the lines. If you've recently had brake work done (that required opening the brake system, simply replacing brake pads does not) than the issue is more than likely some air that did not get bled out.
Brake work that requires opening the system includes: replacement of calipers, wheel cylinders, brake hoses or lines, or the master cylinder.
If no such work has recently been done, then you have another issue, most likely a leak somewhere or possibly a faulty master cylinder. This is especially true if you've recently experienced a "hard brake" or "panic stop" situation in which you applied significantly more pressure to the brake pedal than usual.
Such issues are serious and should be looked into immediately. Even if you can stop, or can pump the pedal to stop, if you have such a problem, you won't be able to stop forever.
From personal experience, the time that you most need to stop will be the time that you cannot. Get it looked at right away.
