A SteamID uniquely identifies a Steam account. This is useful as one can trace back any account via the SteamID, which remains static throughout the lifetime of an account.
A SteamID can have different formats such as SteamID2, SteamID3, or SteamID64.
Many server communities use SteamIDs as identification and might request these in various different formats that findsteamid.com can provide you with.