That is the time, in hours, that a specific aircraft model will take to fly that route round-trip (i.e., frequency = 1.0).
If you see, for instance, 4.80, that means 4 hours and 48 minutes.
Each plane can fly 24 hours a day. How many hours each of your planes has "free" is shown under the column "Hours" in the "View aircraft" screen. Just be aware that the number shown there is rounded up or down to the nearest whole hour. So, a plane with 0.6 hours would show as 1 hour free. This can be misleading. Assume half an hour less than what is shown and you'll be safe.
So, say you have an aircraft showing 4 hours free. If you research a route and, for that plane type, it shows a block time of up to 3.5 hours, you could rent the needed gates and set up the route.
