A shielded operation (or shielded flight) is where your drone remains within 100 metres of, and below the top of, a natural or man-made object. For example, a building, tower, or trees.
When flying shielded you can fly at night. You can also fly within controlled airspace without Air Traffic Control clearance. This is because other aircraft are unlikely to be flying so low and close to structures.
If you’re relying on a shielded operation within 4 km of an airport, then in addition to remaining within 100 metres of and below the height of the object providing the shield, there must also be a physical barrier. A barrier like a building or stand of trees between your drone and the airport. This barrier must be capable of stopping your drone if it loses control.
Before conducting a shielded operation, make sure you familiarise yourself with the rules in Part 101.