A set of fields combinations which could be considered for Primary Key.
In Relational Databases, a “Candidate Key” is basically a (set of) field(s) inside a table that could unique identify one row (a.k.a. ”Tuple”)… but isn’t necessarily being used to do that, functionally. Any given table may have multiple Candidate Keys, but only one of them may be chosen as the ”Primary Key“.
Candidate Keys reveal the possible ways in which a DBA could positively identify a single Tuple. One such way will be chosen & blessed as the “Primary Key”. Other sets that would also function are all considered “Alternate Keys”. Candidate Keys are just the Primary Key and all sets of possible Alternate Keys.