Declension in English occurs when we change the form of a word to show its function in a sentence. It's like giving the word a job title so we know how it fits in with the other words around it.
For example, let's use the word "cat". When we talk about one cat, it's just "cat". But when we talk about more than one cats, we add an "s" to the end to show that there's more than one - "cats". This is called plural declension.
Another type of declension is possessive. This is when we show that something belongs to someone or something else. For example, if we talk about the cat's toy, we use an apostrophe and "s" at the end of "cat" to show that the toy belongs to the cat.
Overall, declension is just a fancy word for changing the form of a word to show its role in the sentence. It helps us understand the meaning and relationships between words.