can I get an explanation on this sentence on options on futures?

From the section in volume six about options on futures:

“If the holder exercises the call and establishes a futures position, he earns interest on the futures margin account.”

I’ve never had a margin account so trying to figure this one out.

Futures have margin accounts that pay interest. If the margin is, say, 50% and you have a $100,000 futures position, then you have to post $50,000 margin. (Most people post this in T-bills, which earn interest.)