adjust it a few ways, one is if you have an adjustable bridge you can adjust it like that (be sure to lower the pickups if you already have them quite hight too).
another is to adjust the trus rod, tho probably not a good idea as if you adjust it incorrectly u could screw your guitar up.
and the last way, is well similar to the first way, if you have a fender style bridge then you can adjust it there somehow but its fidgety i believe.
low action = faster fingering of the fretboard/ easier fingering of the fretboard.
disadvantage = you may get fretbuz if you lower the action to much. some guitars are designed more with low action in mind then others (ie guitars made for metal and soloing) - they aren't as likely to buzz because the frests are probably smaller or the guitar is just set up differently.