Pelasgians were a group of people who lived a really long time ago, even before the Ancient Greeks! They lived in places like Greece, Italy, and Turkey.
Imagine if you and your friends all spoke the same language and had the same way of doing things, but the people in the next town over spoke a different language and had different customs. That's kind of how it was with the Pelasgians and the Ancient Greeks.
We don't know a lot about the Pelasgians because they didn't write things down like the Ancient Greeks did. But we do know that they were very important because they helped create some of the earliest foundations of Western civilization.
Even if we can't see them anymore, we can still see the influence they had on the world around them. For example, some of the architectural designs they used can still be seen in buildings today!