Grammatical case is like a special hat that a word puts on to show what job it is doing in a sentence. Just like when you play dress-up and put on different hats to pretend to be a fireman, a princess, or a cowboy, words also wear different hats to show if they are the subject, the object, or something else in a sentence.
So, when you say "I went to the store" the word "I" is wearing the subject hat because it's doing the action of going to the store. But when you say "The dog chased me," the word "me" is wearing the object hat because the dog is doing the action of chasing "me."
There are different types of hats, called cases, that words can wear in different languages. Some languages have more cases than others, but they all work the same way: to show the role a word plays in a sentence. So, next time you read or speak another language, try to spot the different hats that the words are wearing!