If you have mipmaps then GPU automatically selects one of mipmap when applying texture to polygon. Bias parameter can offset the mipmap number.
If bias is 0.0 then current mipmap is used.
If bias is 1.0 then smaller bitmap is used.
If bias is -1.0 then larger mipmap is used.
Of course values like 0.5 or -3.5 are also allowed.
A little performance hint - calling texture2D with bias specified (even if it’s 0.0) is much slower than calling texture2D without bias. This is true on GeForce6 - I’m not sure about other GPU’s.