I'm fairly new to database management and this question never seems to be answered in more than one sentence. All other SO answers say "A candidate key is a minimal super key." That means nothing to me.
A candidate key is supposed to specify uniqueness of a db record, correct? And a primary key is a candidate key. If a primary key already specifies uniqueness, what's point of adding more candidate keys?
I have seen example records like the following:
Employee(ID, Name, PhoneNumber)
where ID is the primary key and PhoneNumber is a candidate key. From what I see, the ID is enough to specify the uniqueness of an employee record. Although PhoneNumbers are (probably) unique, specifying them as a candidate key does not seem "minimal" to me.
It means that if PhoneNumber was indeed a candidate key you could delete the ID column and use PhoneNumber instead. In other words, it is a candidate for being a unique key.
Wikipedia has a more formal definition that you many want to look at.