A group of words which expresses a complete idea or thought is called a sentence.
He bought a car.
The above group of word expresses a complete idea. Such combination of words is called a sentence.
Now, look at another group of words “book a read him”. This group of words does not make a complete sense so it cannot be called a sentence.
A sentence is a combination of words including a subject and a verb which express a complete idea. A sentence begins with a capital letter and ends with a period, question mark or exclamation mark.
He goes to school.
She is eating an apple.
My name is John.
What are you doing?
Who are you?
What a beautiful flower!