When you have recorded speech or vocals for a song, you generally don't want anything but those speech or vocals. Regardless, you often end up with breathing and other noise you don't want. If you have a decent recording to start with, a noise gate is a simple tool that can help you remove unwanted background noise — like breathing — and it's something that generally comes free with any audio editing application.
Using a noise gate is really easy, especially if you're using a simple application like GarageBand. Garageband simply requires you turn the noise gate on for the track that contains your recording and then set the threshold of the noise gate anywhere from -100dB to 0dB. The threshold is basically the softest your audio track can get without being silenced by the noise gate. If you set it to -10dB, pretty much everything would be silenced unless it was fairly loud. If you set it to -100dB, it would be like the noise gate wasn't really even active in the first place.
In more complex audio-editing software, you'll find the noise gate has a few more settings. One common setting is the speed in which the noise gate reacts. If a noise gate reacts very quickly, it will immediately silence all sounds. This can sometimes be pretty jarring because you'll move from an audible noise to complete silence in a very, very short amount of time. A slower noise gate will reduce the volume over a set number of milliseconds, fading from the loud point in the audio to silence. This is much more pleasant, but if the noise gate is too slow you'll hear the fade and it can sound bad as well. Garageband seems to pick the speed by itself and does it reasonably well, but if you're picking the speed of the noise gate yourself you'll want to find a happy medium between fast and slow that sounds good to you.
Noise gates are generally free plug-ins for most audio editing applications, but if you have one that uses VST plugins, you can download several free VST noise gate plugins. It's the sort of thing you'll get the hang of better by messing around with the settings, but to get a basic idea of noise get functionality and what it does, be sure to watch the video above.