Airtime is negative lateral Gs. This is that weightless feeling you get from coasters. Think of it as your body wanting to go up, but the coaster is travelling down -- like the coaster is trying to eject you from the seat, but the restraints hold you in. The easiest way I know of to achieve airtime in Planet Coaster is with up and down hills at a decent speed. You don't want sharp " /\ " style hills, but instead you want humps. A smooth incline followed by a smooth decline with no flat parts on top.
Planet Coaster records two stats for airtime. The first stat listed is how many times your coaster achieves that airtime feeling and the second stat is how many seconds in total the feeling lasts. So, in theory, you could achieve airtime ten times on a coaster, but it only total one second in total duration. Or, you could achieve airtime once for a total duration of five seconds. To gain a higher duration of airtime your humps need to be longer (not higher) and typically faster.
When I completed the goal, my coaster had a total of three humps designed for airtime. My stats show I achieved airtime six times (hard to tell where the other three were) for a total of 6.6 seconds. I was using the Mine Cart Coaster, chain lift ~150ft, large drop, two bunny hops (humps designed for airtime), helix up to make a U-Turn, another drop, another bunny hop, and back to the station.