What it is
A fingerprint is derived from properties of the connection that are hard to vary casually: the exact sequence and timing of packets, protocol quirks, how the client responds to particular server states, what it claims about itself and whether that claim matches its behaviour. Combined, these produce an identifier that stays the same across connections from the same software.
Why it matters for Minecraft
Because IP bans lost their teeth. An attacker with a pool of residential proxies can present a new address for every single join, which makes per-address enforcement a treadmill. Fingerprinting moves enforcement onto something the attacker would have to re-engineer rather than merely re-lease — banning the tool instead of its current disguise.
What it is not
It is not a Mojang account, and it is not a device identifier. Two players running the same client build share characteristics, so a fingerprint is evidence rather than proof and belongs alongside other signals rather than as a sole basis for a permanent ban. Used well it is decisive against automated tooling and quiet for ordinary players.
Common questions
Can a fingerprint identify a specific person?+
No, and it is not meant to. It identifies client software and its behaviour, not an individual. Many legitimate players share a fingerprint simply by running the same launcher and version.
Does fingerprinting break for modded clients?+
Mods change client behaviour, so they change the fingerprint — which is expected and fine. What matters is that a given client build is consistent with itself, not that it matches vanilla.