Ascension of Jesus means that after he died and came back to life, he went up to heaven. Remember when you fly a kite and it goes up higher and higher until you can barely see it? That's kind of what happened with Jesus. He went up and up until nobody could see him anymore. His friends who were watching were sad but also happy because they knew that Jesus was going to be with his father in heaven. It's like if your mom or dad have to go on a trip but you know they're going to come back soon. So, the ascension of Jesus means that he's still watching over us from heaven and it's kind of like he's always with us even though we can't see him.