+ve air chamber is basically the main spring (which holds you + weight of bike up)
-ve air chamber helps the small bump movement by acting against the +ve air spring pressure to make the forks more reactive (compress easier), it holds less air (by volume) than the +ve chamber so only helps for the first part of the travel i.e. over small bumps
Both chambers should have roughly the same pressure in them for a lively, smooth fork, for a stiffer fork put more in the +ve than -ve.
Note that more pressure in the forks will require more rebound damping as they will move out faster, teh rebound damping is designed to slow the extension of the forks after compression.