This is a bat detector. It detects sounds emitted at frequencies inaudible to the human ear and translates them into sounds we can hear. This evening we detected and saw about half a dozen common pipistrelle bats. If you see a bird flying at night, it’s a bat (unless it’s an owl*).
*We’re pretty sure there aren’t any other nocturnal birds in the UK, but please do correct us if we’re wrong.