What does the maximum supply of a coin actually tell me?

Maximum supply is the ceiling on how many coins can ever exist under the current protocol rules. It tells you the upper bound of emission, nothing more. A capped supply means new coins stop being created once the ceiling is reached, after which miners are paid only from transaction fees. What max supply does not tell you is value, demand, or how the coins are distributed today. A coin can have a low max supply and still be widely available, or a high one and be concentrated in few hands. Treat max supply as a fact about the protocol, not a signal about price. Always confirm the figure against the chain rather than a marketing page.