And what is inflation?

A general, continuous increase in the overall price level of goods and services in an economy over time.

Not really related to the discussion here though, since we were determining whether the interest rate is zero.

Any interest rate that is equal to or less than inflation is functionally zero (or less).

seems like a weird hill to die on when we're discussing the nominal rate, rather than what someone considers to be a "functional rate"

That is also true.

But if they pay you roughly the inflation rate as interest, doesn’t that mean the use of your money is roughly free to them?

Yes, and alternatively you could invest your money in whatever they'd invest in, and capture more value. You could even invest in risky things like stocks with your money, whereas they can't (with your money).