On your fleet page you can manage all your aircraft. You can switch between your different fleets, the unassigend aircraft (not assigned to a fleet) and your aircraft which is leased to other airline. For each aircraft there is a green/orange/red dot shown which indicates that your aircraft is either free for booking, currently enroute or in maintenance/transfer/currently booked. Just hover over the dot to see the reason for the current color.
On the top of the page you find some links (if you have the fleet manager rights) to manage your fleets.
Create a new fleet.
Delete the currently selected fleet.
Rename the currently selected fleet.
Assign aircraft (unassigend or from another fleet) to the selected fleet.
Here you can buy or lease aircraft.
Next to each aircraft are some buttons (if you have the necessary rights) where you can manage the individual aircraft.
Next to each aircraft is a info button which leads to the aircraft info page.
By clicking on this button you can repair your aircraft.
Here you can transfer your aircraft to another location. Transfers aren't performed instantly, the usual flight time for this distance is taken to determine how long the transfer takes.
This is where you can sell/lease/scrap your aircraft.