as you download from the internet (by downloading, i just mean getting stuff from the internet. so this includes reading your email, browsing website, anything. including things you'd normally call "downloading", like music), the service provider (ISP) keeps a track on how much you're getting.
capped services are those that once you have got a certain amount of data, either stop working or (more commonly) begin to charge you for the extra stuff you're downloading if you go over your monthly limit.
uncapped ones, or unlimited ones (you may have seen "unlimited broadband" from AOL), are those that have no limits, or no cap. you can get as much as you want from the internet, they don't mind.
if you only want the speed of broadband, and will probably just be browsing the web a bit, checking your email, then a capped plan would probably not interfere in your daily business. but if you use the internet a lot, and do a lot of downloading (music, games, photos, or just lots and lots of internet browsing), then the cap may affect you, and you'd be best going for one with no cap.
said that, as wideboy says, it's best to try and find an uncapped service anyway.