well, I know that the numbers represent the fret, and the horizontal lines represent the strings.

what else should i know?
I wanna go back to where I was...
er... i forgot to mention that. thanks.
anything else?
I wanna go back to where I was...
Sometimes you might get a rhythm written above the piece, and this should have a key at the bottom so you can understand it.
Founder of Jaco society

[22:08:23] <Confusius> I wish I was a bassist
[22:08:26] <Confusius> you fuckers look cool

Want to know how to play bass in jazz? Read this.
Basically tab is paint by numbers. You need to listen to the song (see what the picture is of), hear how the bassline goes (look at all the colours), and then play it (paint it) with numbers (numbers). You have the basic tools to play tab; bass, amp, cord, tab etc. (canvas, paintbrush, colours, numbers etc.) so go play some. Whew, that was a lot of analogizing.

Here's a great explanation on how to read bass tab. Even goes into how to read tunings, rhythm and other symbols in tab.

However, don't just limit yourself to reading tab. You should start getting down some music theory (including reading notation for which there's a good sticky thread in the forums right now). I would recommend getting a teacher if its affordable; and at the very least getting a good bass book--Hal Leonards Bass Method I - III is the one I usually recommend.

And look at the powertab software on UG--it has the advantage of letting you look at both the tab and notation for songs and you can "play" along with it. Its a free alternative to Guitar Pro (which if you do it legally does cost some bucks).

Good luck and welcome to the forums!
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what else should i know?

To use the search bar.
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I live by the method: 3 or less orange warning labels, and it's safe as a kitten

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EDIT: Sammcl pretty much got it dead on.