Sometimes, Git can't make your change to a remote repository without losing commits. When this happens, your push is refused.

If another person has pushed to the same branch as you, Git won't be able to push your changes:

git push origin master
# To https://github.com/user/repo.git
#  ! [rejected]        master -> master (non-fast-forward)
# error: failed to push some refs to 'https://github.com/user/repo.git'
# To prevent you from losing history, non-fast-forward updates were rejected
# Merge the remote changes (e.g. 'git pull') before pushing again.  See the
# 'Note about fast-forwards' section of 'git push --help' for details.

You can fix this by fetching and merging the changes made on the remote branch with the changes that you have made locally:

git fetch origin
# Fetches updates made to an online repository
git merge origin branch
# Merges updates made online with your local work

Or, you can simply use git pull to perform both commands at once:

git pull origin branch
# Grabs online updates and merges them with your local work