This is short for “tarmacadam,” which differs slightly from macadam.
Wikipedia defines it as:
Tarmacadam is a road surfacing material made by combining crushed stone, sand, and tar […] It is a more durable and dust-free enhancement of simple compacted stone macadam surfaces…
The name comes from a specific tar-based paving material that’s also commonly used on roads. Originally, the word was trademarked as shorthand for tarmacadam, “tar mixed with crushed rock…”
So, it technically refers to a specific method of road construction, but honestly, we know it as the concrete areas at airports.
I gather that in the UK, “tarmac” is sometimes used as a verb, like we would use “pave” here in the US.
We should tarmac that gravel road.