Kalbom is an extra amount given when a whole shekel is given to the Bais Hamikdash. This amount was added by the chachamim for irregularities in the coins.

The following people must give a Kalbom: Leveiim, Yisroailim, Proselytes, and freed slaves, but not Cohanim, women, slaves, and children. If a person gives shikalim for himself and a Cohein, woman, slave, or child he does not have to bring a Kalbom. If he gave for himself and a friend, he is only obligated in one Kalbom. R’ Mayer says he must bring two Kalboms.

If you give a selah and get change of a shekel (a selah is equal to two shikalim) you must give two Kalboms.

Shikalim 1:7

If a peron gives Shikalim for a poor person as a friend and does not plan on making him pay back, he is not obligated to bring a Kalbom. Partners that are obligated in a Kalbom when they give Shikalim, do not have to give Maaser Behaima (the 10^th animal) since partners are not obligated in Maaser Bihaima.

A Kalbom is worth 1/12 of a ½ a shekel according to R’ Mayer. The Chachamim say that a Kalbom is worth 1/24 of a ½ a shekel.