There are two islands, Sky Isle (Day) and Sky Isle (Night). You can choose to enter either one.
There is a countdown timer for the Nimbus Islands. Once the countdown ends, you will be transported to somewhere high up in the Playzone.
While on a Nimbus Island, you will not suffer damage from the Blue Zone. The Nimbus Islands disappear at the end of the countdown and will not be available again in the same match.