The reference is to individual decisions/actions. In the letters to the seven churches, the Lord repeatedly found fault in his hearers’ conduct and called them to repentance (Revelation 2:5, Revelation 2:16, Revelation 2:22, etc.). He also caused the gospel to go to the nations with the imperative to embrace that eternal gospel (Revelation 14:6). How people have responded to imperatives as these forms part of the content of the phrase what he has done.
More contextually, Jesus hooks onto the activities of the previous verse, where the evildoer does evil and the righteous does rightly (Revelation 22:11). What one has done
in response to such instruction now forms the basis of the reward the Lord will give. See also Revelation 20:12.
12 “Behold, I am coming soon, bringing my recompense with me, to repay each one for what he has done.