Monitoring refers to listening to audio through a reference playback system during recording or mixing. The goal is an accurate representation of the audio so that decisions made during the session translate correctly to other playback systems — car speakers, earbuds, phone speakers, and hi-fi systems.
Studio monitors (speakers designed for flat, accurate frequency reproduction) and reference headphones are both used for monitoring. Each has advantages: monitors reveal stereo imaging and provide a more natural listening experience; headphones are unaffected by room acoustics and provide better isolation of individual elements.
Direct monitoring is a feature in audio interfaces that routes the microphone signal directly to the headphone output with near-zero latency, bypassing the computer. This allows the performer to hear themselves without the delay that would otherwise make singing or playing in time very difficult.
Professional mix engineers often check their work on multiple monitoring systems — high-quality studio monitors, regular consumer speakers, earbuds, and in a car. This process (called 'referencing') ensures the mix translates consistently across a wide range of listening environments.