What is Complement ?
Complement is (noun) 1. a thing which adds to or fits in with something else Mint sauce is the perfect complement to roast lamb. 2. a ship’s complement the full crew of a ship 3. a(noun) or adjective which follows the verbs ‘be’ or ‘become’ In the sentence ‘he’s a big boy’ the words ‘a big boy’ are the complement of the(verb) ‘is’. (verb) to fit in with something The two colours complement each other perfectly. Her jewellery complemented the colour of her hair. (NOTE: Do not confuse with compliment.)
source: Easier English, Student Dictionary Upper Intermediate Level