In tablature, each line represents a string, and each number represents the fret you hold the string on. A simple four line staff…
Tabs:
G|-----------------|-----------------|
D|-----------------|-----------------|
A|-----------------|-----------------|
E|-0-0-0-0-0-0-0-0-|-3-3-3-3-5-5-3-3-|
…represents a 4-string bass. Imagine you're looking down at your bass as it lays face-up on your lap. The bottom string of the tab is the top (aka 4th, aka “low e” ) string of your bass. The letters at the beginning of each line represent the tuning. EADG is standard bass tuning.
In the tab above, “0” represents an open string (meaning you're plucking the string without pressing any frets). 3 is the 3rd fret; 5 is the 5th fret, etc. The notes in the tab above would be E (open E string), G (E string, 3rd fret) and A (E string, 5th fret).
Pay attention to legends, as well. h = hammer on, p = pull off, / = slde up, \ = slide down, etc.
Any more questions, feel free to ask. We're here to help.