A morpheme is the smallest part of a word that helps us understand what the word means. Morphemes give meaning to words. Let's take a simple elementary word like dog. When I see the word dog, I understand it to mean one animal- one dog. But if I add an S to the word dog, now I have "dogs," meaning more than one dog. That "s" is a morpheme because it helps others understand that I'm talking about more than one dog.
Let's look at another simple word: fish. Now, that word means one fish. But if I added "es" to the end of that word, now it means more than one fish. I've changed how you think about that word by adding a pair of letters that make the word plural. That pair of letters "es" is a morpheme because it tells us that the word means more than one fish. But "es" can also mean present tense action. Adding "es" can change the word to a verb that means something that occurs repeatedly: He fishes here every Sunday.
Now, if I add "ed" to the word fish, I have to think of the word fished as a verb and an action that has taken place in the past. Adding "ed" to the word requires others to understand the word as an action already taking place. "Ed" is a morpheme because it is the smallest part of the word that tells me how to understand it. "Ed" adds a new meaning to the word fish.
But what if I added a Y to the word fish? Now I have an entirely new word with new meanings: fishy. Fishy can mean something that smells like fish, or it can be an idiomatic expression meaning that something is suspicious. "There's something fishy about this situation," said the detective. So, "Y" is a morpheme because it is the smallest part of the word that affects its meaning.