The VBA Help blog

Sometimes you don't know until run time which method to call ... the name's sitting in a cell, a file, a registry key or a button's Tag. VBA's CallByName turns that String into a call: getting or setting a property, or calling a method, by name.

Most of the procedures you write are called by you, or by VBA on your behalf. A callback turns that around: you hand another piece of software the address of one of your own procedures and let it do the calling. How to do this in VBA is with the AddressOf operator.