Imagine you are looking down at the fingerboard of a guitar or banjo from above. Tablature shows a line for each string with the first (highest sounding) string at the top and the lowest sounding (bass) string at the bottom. Each fret is given a number, 0 for unfretted, 1 for the first fret, 2 for the second and so on.
Each string of the guitar or banjo is shown as a line on the tablature
But how long do you play a string for?
There are symbols above the fret number which tell you how long to hold down a string.
This means hold down the string for one count
This means play the string twice for half a count each
Numbers arranged on top of each other mean play these strings together as a chord
Don't worry about learning all the different symbols now! This is just to give you an idea of how it works. But if you want to know more, here is a more detailed explanation at the excellent clawhammerbanjo.net site.