SteamID64 is one of the multiple ways steam provides to uniquely identify a Steam account, and it's the newer format of Steam ID's that are used by games, software, and mods to identify players that are logging into a Steam based server. You'll often need to find your, or another players,
SteamID64 in order to configure a server to treat that Steam account differently in some way. This can be adding them as a Server Admin, adding them to a Whitelist, or running in-game commands that need to specify the user they act on.
You can find a Steam accounts
SteamID64 using SteamID.io and entering the information you already have about the account, such as their username or account URL, into the search box. After you get the result you're looking for, you want to click the copy icon on the far right of the
steamID64 row. That will place the
SteamID64 of the account you found into your clipboard and you'll be able to use it in whatever system you're setting up.