When Zapier sends a request to another app, it will wait for an OK/error response before moving on to the next step. But, it can't wait forever - actually it usually only waits for 30 seconds. If after that time, it's not received any response from the app's API, it assumes there has been a problem and sends you an error message.
What causes a Zapier timeout error?
99% of the time, it's caused by temporary downtime at the app Zapier is connecting to. Or if the API is receiving too much data from other apps.
However, two things might have happened:
Nothing - the app Zapier is connecting to is down and what you wanted to happen, didn't happen.
The action you wanted to happen (like adding a new row to a Google Sheet) was successful, but the app's API wasn't able to send a response message to Zapier, so you got an error.
How to fix a Zapier timeout error
You'll want to check the Zap History, because if it's the former of the above options, you might not want to do anything. Also, if nothing shows in the Zap History then most likely the timeout occurred at the trigger level and will pull what was missed at its next polling interval.
If the error is showing in Zap History, you can replay the zap.