Pace in a story means how fast or slow the events happen. When the pace is fast, it feels like things are happening quickly, like when you run or ride a bike really fast. When the pace is slow, it feels like things are happening slowly, like when you're walking or sitting still.
In a story, the pace can change depending on what's happening. When there's action or excitement, the pace might be fast. When there's something sad or thoughtful, the pace might be slow. Sometimes the author wants to keep the pace the same the whole time, so you feel like you're in a certain kind of mood while reading the story.
It's kind of like watching a movie. Sometimes the movie is really exciting and you feel like your heart is pounding. That's because the pace is fast. Sometimes the movie is sad and you feel like crying. That's because the pace is slow. When you read a book or hear a story, it's the same thing - the pace can make you feel different feelings, just like a movie can.