Exercise: fighter jets perform mock interception of Romanian C-27
https://www.nato.int/cps/en/natohq/photos_194762.htm
Spy planes are conducting missions to track Russian movements, which might include listening to Russian soldiers' phone calls home. The planes are equipped with devices that allow them to collect signals sent from Russian bases and to intercept communications.
Analysts stationed on the plane and back at base can then assess the information gathered and turn it into "intelligence" about Russian movements and capabilities.
The position of the planes also helps reveal where NATO countries are interested in and the position of Russian warships - a circling aircraft at sea suggests they are interested in something at the location.
On board a Nato surveillance plane monitoring Russian activity in Ukraine - BBC News