Original: A pickup device acts as a transducer that captures mechanical vibrations (usually from suitably-equipped stringed instruments such as the electric guitar, electric bass guitar or electric violin) and converts them to an electrical signal, which can be amplified and recorded.
Simplified: A pickup is a device that captures mechanical vibrations (usually from stringed instruments like the electric guitar, electric bass guitar or electric violin) and converts them to an electrical signal which can be amplified and recorded.