This command can be used to look up information about a user's profile. It can look up their name, profile picture, user ID, and when the account was created.
If the user you are looking up is in the server you are executing the command in, it can also look up when they joined the server and what roles the user has.
If you use user IDs, the bot will be able to find any user on Discord, even if they are not in a mutual server with the bot. Not giving the bot a user to look for will simply return your own user information.
If the bot fails to find the user ID you gave it for whatever reason, it will also return your own user info.