The Binding of Isaac temp

ETA=Estimated Time of Arrival.
For example, if you're on a way to someone's house for a party, and they ask you what your ETA is, then they're asking what the estimated time you'll get there is.

Or like, if you checked google maps and it said that you would get to the party at about 3:31 PM, then you would tell them that your ETA is 3:31 PM

ETA is basically if you were going at [X] speed, at 8:00 as a example, then it would make a prediction on when you would arrive at a destination.

when will you arrive?

It means, 'What time will you get here about?'