A group of words that expresses a complete thought is called a sentence. A sentence must have a subject (who or what the sentence is about) and a verb (what the subject does or is).
Here are a few examples of sentences:
* The cat sat on the mat. (Subject: cat, Verb: sat)
* The sun is shining. (Subject: sun, Verb: is shining)
* I love to read. (Subject: I, Verb: love)
If a group of words doesn't have a subject and a verb, or if it doesn't express a complete thought, it's not a sentence. It might be a phrase or a clause.