Okay kiddo, so you know that English is a language, right? Well, a really long time ago, there was a kind of English called Old English. It was spoken by people who lived in England a thousand years ago, so it was very different from the English we speak today!
Now, Old English had a lot of words that we don't use anymore. Some of them are words for things that don't exist anymore, like "cattle-reek," which means the bad smell that cows give off. Others are just really old-fashioned, like "yfle" instead of "evil" or "hors" instead of "horse."
As time passed, people stopped using these old words and started using new ones instead. So some words disappeared completely, while others changed into the words we use today. For example, "cyning" (=king) became "king," "eorl" (=nobleman) became "earl," and "writan" (=to write) became "write."
So basically, over a long time, people stopped using old words and started using new ones, and that's why Old English vocabulary is different from the English we use today.