what does a complement mean ?

A grammatically correct sentence consisting of at least one verb. But, all sentences are not this simple. In most sentences, additional words follow the verb. These words are known as complements. Thus, a complement is something that completes. There can be two kinds of complements:

1. Subject complement: A subject complement completes an idea about the subject and gives us additional information about it. A subject complement is usually a noun, a pronoun or an adjective. A subject complement usually follows a linking verb ( such as 'be', 'become' etc) which links the subject with the subject complement.

For example:  Parul is a good teacher.

'Parul' is the subject and 'a good teacher' is the subject complement connected to the subject with the help of a linking verb 'is'.

[Note: the subject is not doing anything in the sentence except being something]

2. Object complement: It renames or describes an object and can be a noun or an adjective.

 For example: I found the watchman sleeping.

Here, 'watchman' is the object and 'sleeping' is the object compliment.