Do you know how sometimes people use different words or have different accents depending on where they come from? Well, a long time ago, there were a group of people who lived in Ireland and they talked in a different way than people do today. They had their own special words and ways of speaking that are different from the way people speak now.
This special way of speaking is called the Goidelic language. It's also known as Gaelic or Irish Gaelic. When other people came to Ireland, they started talking with the Goidelic people, and over time, some of their words and way of speaking were added to the Goidelic language.
The Goidelic Substrate Hypothesis is an idea that some linguists (people who study languages) have. They think that some words and patterns in the English language can be traced back to the old Goidelic language. This means that even though we don't speak Goidelic anymore, some of its words and ways of speaking still exist in the English language.
For example, the Goidelic people had a special way of saying the letter "h" that was different from the way other people said it. They would say it like "ch" instead of "h". Over time, this way of speaking may have influenced how some English words are pronounced.
So, even though the Goidelic language isn't around anymore, it still has an impact on the way we speak today.