$ man merge-conflicts

Merge Conflicts

When Git can't automatically combine changes from two branches because they edited the same lines.

This is the tax you pay for collaboration. Two people edited the same file. Git doesn't know which version to keep. You have to choose manually.

Open the file, look for the conflict markers (<<<<<<< and >>>>>>>), decide which code to keep, delete the markers, commit the resolution. It's annoying but necessary.