Whenever local repository is updated with changes in its remote repository with a git push command, the local repository is updated with the changes in the remote repository with the git pull command, or one branch is updated with changes in another branch, git attempts to automatically merge the changes into the destination repository or branch.
Most of the time, this merger is successful and nothing else needs to be done. Sometimes, there are conflicts with the changes. This often occurs when two programmers are working on the same branch in the same section of code. Git does not try to resolve these conflicts because it cannot tell which changes are more important than others.
It is up to the programmer to resolve any merging conflicts manually. Git will mark the conflict by leaving both sets of changes in the source code. The two conflicting sections of code will be between the following three lines:
The merge conflicts will be in the files in the local repositiory. When merge conflicts occur, the programmer needs to search for these conflicts and resolve the code so that the two conflicting sections become one section.
Once all the conflicts are resolved and commited to the local repository, the git command that found the conflicts needs to be rerun.
See the Git CLI Commands lesson for details on the git commands listed above.
The following links provide more references to merging:
Git - Basic Branching and Merging