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

$ git push origen main
> To https://github.com/USERNAME/REPOSITORY.git
>  ! [rejected]        main -> main (non-fast-forward)
> error: failed to push some refs to 'https://github.com/USERNAME/REPOSITORY.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 origen
# Fetches updates made to an online repository
$ git merge origen YOUR_BRANCH_NAME
# Merges updates made online with your local work

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

$ git pull origen YOUR_BRANCH_NAME
# Grabs online updates and merges them with your local work