Inter-satellite service is when satellites communicate and work with each other. Kind of like when you play with your friends and work together to do something.
But instead of playing, the satellites are doing important things for us, like helping us with our phones and internet. They talk to each other and share information, like when you and your friends share toys or snacks.
And just like how your friends can help you when you need them, satellites can help each other out when they need to. They can work together to make sure we have good internet and phone signals all the time.