What would happen if everyone on Earth jumped at exactly the same time?

If every person on Earth jumped at the exact same time, the effect on the planet would be negligible.

While the combined mass of 8 billion people is significant, it is tiny compared to the mass of the Earth. Earth is roughly 13 quadrillion times heavier than the total human population. Even if everyone jumped simultaneously, the amount of energy transferred to the planet would be far too small to shift its orbit, change its rotation, or cause any seismic activity.

Physicists who have studied this hypothetical scenario—most notably the creator of the webcomic xkcd, who performed a famous calculation on the matter—have concluded that the only notable results would be a massive amount of noise from the simultaneous jump and potential localized issues, such as the temporary disruption of public transit systems or infrastructure if everyone gathered in one place to do it.

In short, the Earth would not move, and the planet would continue its orbit exactly as it was before.