Declensions may apply to nouns, pronouns, adjectives, adverbs, and articles to indicate number (e.g. singular, dual, plural), case (e.g. nominative case, accusative case, genitive case, dative case), gender (e.g. masculine, neuter, feminine), and a number of other grammatical categories.
For example, in English, if there is more than one boy, the word must become boys. For example, "My family has two girls and one boy, but my wife's family has two girls and three boys". The form boy is singular, meaning there is only one of the noun, and the form boys is plural, meaning there is more than one of the noun. You must know the noun's singular form if you want to find it in an English dictionary.
Also in English, a noun can change if it owns something. For example, if a toy belongs to one boy, the toy is the boy's toy, but if it belongs to two or more boys, it becomes the boys' toy. Both numbers of the noun have this word case, which is called the genitive case. Word case is when a word form changes depending on what part of the sentence it is.