We could understand the clause very broadly to refer to any action anyone has ever performed (Ecclesiastes 12:14), and that would be accurate. But the clause also has a contextual loading. Specifically, John had just seen a vision of the devil being thrown into the lake of fire and sulfur where the beast and the false prophet were
(Revelation 20:10). The devil had sought to “deceive the nations” and the result was that “they marched up…and surrounded the camp of the saints” (Revelation 20:8–9). That action on the part of earth’s population is certainly part of “what they had done” (Revelation 13:3, Revelation 13:8, Revelation 13:14). With the devil and the beast and the false prophet all “thrown” from the earth “into the lake of fire” (Revelation 19:20; Revelation 20:10), it is only fitting that those who followed the beasts and who had heeded the devil now need to appear before God to give account for their actions.
12 And I saw the dead, great and small, standing before the throne, and books were opened. Then another book was opened, which is the book of life. And the dead were judged by what was written in the books, according to what they had done.