Imagine you are playing a game with your friends and you need someone to keep track of the score. The person who is responsible for keeping score is called the "gabbai" - it's like their job is to make sure everyone knows who is winning and who is losing.
In Jewish communities, a gabbai is someone who helps to organize the synagogue and make sure everything is running smoothly. They might help with things like scheduling when people lead prayers, making sure the Torah scrolls are taken care of properly, helping out with charitable events, and making sure everyone who needs help gets what they need.
Just like the scorekeeper in your game, the gabbai is an important part of the Jewish community because they make sure things are fair and everyone gets what they need.