What is Halonium ion, anti and syn addition?

In the addition of $\ce{Br2}$ or $\ce{Cl2}$, the $\pi$ pair attacks one halogen atom while a lone pair of that atom bonds to the second carbon: a three-membered cyclic halonium ion forms, the other halogen leaving as halide. An addition in which the two new groups end on opposite faces of the former double bond is an anti addition; on the same face, a syn addition.