I don't understand how to use the I2t rating to calculate the fuse most appropriate for a circuit.

The formula as given is I2t = I^2 * t, where I=current surge, t=time duration of surge.

I don't understand how to use this formula at all.

If I have a circuit that we do not want anything more than 20mA going through it, say at 5V, what kind of fuse do we chose (and by the way are there fuse holders for breadboards?).