a) Mortar that has been placed between bricks to hold them together. This is not strictly speaking correct word to use in this context, mortar would be the correct word, or joint filling. (or perhaps applies in the US only) This term is often misused as meaning mortar or joint filling, as repointing is the action of making good and repairing of joints between stone.
b) The action of the verb to point.
Wikipedia foundation.