A closed class of words is a part of speech that cannot add new words easily. Open class of words, on the other hand, can easily add new words.
For example, since pronouns are a closed class, there are fairly few pronouns: I, me, my, mine, myself, you, you, your, yours, yourself, he, him, his, his, himself, she, her, her, hers, herself, it, it, its, its, itself, we, us, our, ours, ourselves, they, them, their, theirs, themselves. Since pronouns are used to replace whole noun phrases, there is no need to have many kinds of pronouns. Instead of saying "The Earl of Sandwich introduced the Earl of Sandwich's favorite food, the sandwich", one uses the pronoun "his" to replace "the Earl of Sandwich's" to make the sentence not repeat itself when it doesn't have to, thus the sentence becomes "The Earl of Sandwich introduced his favorite food, the sandwich". New nouns, on the other hand, can easily be made, and are constantly being added into the English language.