Imagine you have a bunch of letters and you can use them to make different words like cat, dog, and hat. Now imagine that each letter has a special job or meaning. For example, the letter "c" in cat means that the word is talking about a kind of animal with four legs and fur.
In grammar, we use the term "grammeme" to talk about these special jobs that letters (or groups of letters) play in making a sentence. Just like the letter "c" has a special job in the word cat, other parts of a sentence like verbs or adjectives also have special jobs or meanings. By understanding grammemes, we can learn how to put words together in the right way to make a sentence that makes sense.