You
Account
Who you are here, and which Discord account is allowed to speak for you.
Discord
Linking lets the bot answer for you at the table: read your sheet, roll with your own modifiers, and mark damage from Discord. Until you link, the bot can look things up in the rules but cannot see a character.
Linked to
One Discord account per member, and one member per Discord account. Linking a second replaces the first, and unlinking takes the access away immediately.