It has something to do with commit signing (https://help.github.com/en/articles/signing-commits).
In case you use Github Desktop (which doesn’t support commit signing as highlighted right at the beginning of the article linked above) or if you don’t need commit signing (or don’t even know what it is), one thing you can do is to disable gpg commit signing by running a command like git config --global commit.gpgsign false or edit your .git/config file manually:
[commit]
# https://help.github.com/articles/signing-commits-using-gpg/
gpgsign = false