Okay kiddo, so a really long time ago, people in England spoke a language called Old English. And just like how you sometimes have trouble saying words or letters, the people who spoke Old English had their own way of pronouncing things.
In Old English, they had some sounds that we don't use anymore in English today. For example, they had a letter called "þ" which made a "th" sound, but it was different from the "th" we use now. They also had a sound called "ð", which was like a softer "th" sound.
They also used to pronounce some letters and words differently than we do now. For example, the letter "g" was sometimes pronounced like the "gh" in "rough". And they would sometimes add little sounds at the end of words that we don't hear anymore.
But over time, the way people spoke in England changed. People from other places came to England and brought their own languages and ways of speaking, and that influenced how English sounded too. So Old English is different from how we speak today, but it's still really cool to learn about!