What does it mean that push git cannot be quickly merged?

I assume you see this problem:

 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to '/Users/mayoff/t/test/central'
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.

Here, as the “non-transitional updates were rejected” problem, the problem occurs.

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, :

( # 4) (commit # 3), , :

(git fetch, git merge) (git pull).

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