Tone refers to the pitch of the sound the metal detector provides when it detects a metallic object. Most metal detectors will have at least two tones to differentiate between ferrous and non-ferrous objects.
The pitch, speed, and other characteristics of provided tones can help with fast identification of possible target types and location upon detection. However, a tone’s volume is only as loud as the signal’s strength, hence the need to be able to hear weak and faint tones which is better acquired through headphones.