What is the meaning of Noun subject of sentence?

The subject of a sentence is what the sentence is about.

A noun is a Word for a person, place, or thing.

The subject of a sentence will be a noun or a pronoun (a pronoun takes the place of a noun)

Examples:

The door has been painted. (the subject of the sentence is 'door', a noun)

My mother made a cake. (the subject of the sentence is 'mother', a noun)

Paris is beautiful this time of year. (the subject of the sentence is 'Paris', a noun)

I forgot my book. (the subject of the sentence is 'I', a pronoun that takes the place of my name, a noun)

They bought a new house. (the subject of the sentence is 'they', a pronoun that takes the place of the names of the people who bought the house; their names are nouns)