Have you ever played a game of telephone, where you whisper a message to someone and they pass it on until it reaches the last person? Sometimes, the message can get a little mixed up by the time it gets to the end.
The Q source is like a game of telephone for the Bible. It's a hypothetical document that some scholars believe was used by the authors of the Gospels of Matthew and Luke. This source supposedly contained the sayings and teachings of Jesus that were not found in the Gospel of Mark - the earliest Gospel.
Imagine that Mark wrote down what he remembered about Jesus, but he didn't write down everything. Then, Matthew and Luke wanted to write their own Gospels, but they wanted to include more of Jesus' sayings that weren't in Mark's Gospel. So, they used the Q source as a reference for what Jesus said.
Scholars think that the Q source was probably written in Greek, the same language as the rest of the New Testament. But, nobody has ever found a copy of it, so we can only speculate about what it contained.
So, in summary, the Q source is a document that some people think was used by the writers of Matthew and Luke to help them include more of Jesus' teachings in their Gospels. It's like a book of notes that Mark didn't have.