Ah check.
A filter is usually a term for a process ( usually a device or software plugin ) that acts as a frequency equalizer. However, I meant a pop-filter

A small screen you place before the microphone that prevents the more aggressive air blowing into the mic. It stops the sound of breathing and bass heavy and or popping sounds usually occuring on pronouncing "b"'s and "p"'s. It also keeps the mic from getting wet with particularly enthusiastic singers.
You could easily make one btw by just taking a pair of nylons and stretching it over a clothes hanger.
Compression is a process - nothing to do with data compression as it occurs with MP3s!! - that basically keeps the levels of the sound more or less even. For instance you could shout one word and then whisper a few others. While this has a great dynamic range - and thus interest to the listener -, if you were to do this over a full backing band chances are you wouldn't be able to hear the whisper, so you turn up the volume of the vocal. This causes the scream to be over the top loud or even to distort. Compression acts as the mediator making sure the volume of both the scream and the whisper are even, so the overall track volume stays more or less the same. Of course, this has to used in context and to taste to prevent a vocal (or any sound really) from sounding "flat" and bland and removing overall impact, ideally you just want an even sound signal that doesn't sound like it has been tampered with. Popular use today is to have a compressor work next to an aggressive element in the mix ( such as the kick in dance music ) where the compressor ducks the signal during the kick and ups it between the kicks. But let's get to the point:
A compressor is either a piece of hardware or a software plugin that acts as a signal processor and I'm sure Garageband has one. A compressor has several parameters, often changing between brands and types, but the most common parameters are:
threshold, when a signal exceeds the threshold the compressor kicks in and the sound is "squatted". Set it so only the loud sounds receive compression if you wish to tame a signal.
ratio, exactly that, how much the outgoing sounds volume is altered relative to the incoming sound.
attack, how fast the compression should kick in after a peak has exceeded the set threshold. This allows you to create gentle fades instead of radical volumes dips, once more use to taste and context
release, the time it takes for the signal to return to the original level once the signal returns to below the set threshold.
It's one of those utilities that doesn't seem too spectacular in itself, but when used in context with other instruments / in a mix it becomes a necessity to get an even balance.
However within the context of the Chet Baker song, the vocal compression shouldn't be too extreme, just look at the meters - or better yet listen to the sound - and make sure the response is even.