SteamID64 is the unique ID number of every Steam user. It should look something like this : 7656119XXXXXXXXXX. I've replace some of the numbers with "X" so I don't reveal my steam profile, but you get the idea.
To find the SteamID64 of someone, you will need to go to their steam profile. The URL will look something like this : https://steamcommunity.com/profiles/7656119XXXXXXXXXX/. Again, I've hidden some of the numbers, but the last part of the URL is the user's SteamID64. Some people, however, will be using a custom steam URL. It will look something like this. https://steamcommunity.com/profiles/example-user/. To find the SteamID64 of a user like this, paste their entire URL into this website.