What's a pithy two or three-word description that someone else might use to sum up your character? A 'concept' as we use it is not intended to be a curt dismissal of the depth to a character, but rather a tool- an extremely simple descriptor that allows players to get a very broad feel for the role of a character so they can help set the right tone for a role-playing scene.
Typically, concepts will be as simple as an adjective that might describe a character's personality, goals or appearance along with a noun for their occupation or notable characteristic that can be used to describe them. "Ambitious Noblewoman", "Amoral Sellsword", "Gregarious Merchant", "Taciturn Mason" would all be examples. A character's concept can and will change as a character develops and grows- someone who originally makes a young, brightly optimistic character might decide to go with "Idealistic Squire" as a concept, then it could become, "Hardened Knight" or "Cynical Veteran" later through roleplaying development.
Characters that have concepts that make more sense to be secret should select a concept that would seem to be the public face of a character, while a separate true concept to represent the inner nature of the character will remain hidden. For example, for someone that would play say, "Courtier Assassin", "Demonic Cultist", "Scheming Manipulator", "Bandit Leader" or the like might instead take, "Charming Courtier", "Amiable Noble", "Demure Lady-In-Waiting", or "Shy Outdoorsman" to hide their actual concepts. Whenever in doubt if a concept is appropriate, feel free to ask staff.