Merging is the operation of taking two or more source contents to produce a single one. Things that are commonly merged are files (source code, configuration, etc.), branches in a version control system, redundant entries in databases, etc.
For example, merging two files that derives from the same original file in two different ways consists in taking the modifications made in both with respect to the original file to integrate them and produce a file where all the modifications from the diverging files appear. Sometimes, merging implies conflict resolution if the modifications are incompatible.