Words put together so as to make sense form what is called a sentence.
‘Barley oats chair really good and cherry’ is not a sentence, because it makes no(n)sense.
‘Tom has said his lesson’ is a sentence.
It is a sentence because it tells us something about Tom.
Every sentence speaks of someone or of something, and tells us something about that of which it speaks.
So a sentence has two parts:
(1) The thing we speak of;
(2) What we say about it.
In our sentence, we speak of ‘Tom.’
We say about him that he ‘has learned his lesson.’