The Fear & Greed index is a single number, from 0 to 100, that captures the emotional temperature of the whole crypto market. It blends together several ingredients — how wild prices have been lately, how strong the momentum is, how much is being traded, and how loud the social-media chatter is — and boils them into one easy reading.
A low number means fear: people are anxious and selling. Historically, readings below about 15 have tended to appear near market bottoms — the moment when almost everyone has given up hope. A high number means greed: people are excited and piling in, and readings above about 80 often show up when things are getting frothy and overheated.
The famous investing line captures the whole idea: be fearful when others are greedy, and greedy when others are fearful.