Bellows factor:

If "m" is the magnification of your subject then you multiply your exposure time by (1+m)^2.

For example, if you want to photograph something at 1:1 (same size on film as in reality), then m=1 and your factor is (1+1)^2= 2^2 = 4.

So if you hand metered a 1:1 macro subject and determined the exposure to be, say, f/16 and 1 sec, then the correct exposure (including bellows factor) would be 4 seconds.

If you are doing portraiture in b&w in 4x5 or smaller formats then bellows factor is unlikely to be a major issue- you never get anywhere near 1:1 On the other hand, it is quite plausible to do 1:1 portraiture with an 8x10 camera or larger. In any case, while you are learning about bellows factor, I (and many others) would strongly recommend using fuji instant film until you feel confident.

This may all sound like greek to someone shooting 35mm or any system that meters TTL (through the lens) but... really it's just a quite simple application of the inverse square law.