Generation time is how long it takes for a new person or creature to be born, grow up and have cubs or babies of their own. It's like how long it takes for a seed to grow into a plant and then make more seeds.
Let's say we have a mommy bunny who has baby bunnies. When the baby bunnies are born, they are really small and can't take care of themselves. They need their mommy to feed and take care of them. As they grow up, they learn how to eat on their own and eventually become big bunnies.
Once the baby bunnies become big enough, they can have their own babies. That's called their generation. It takes a certain amount of time for the baby bunnies to grow up and have babies on their own. That time is called the generation time.
So, in summary, generation time is how long it takes for a new animal or creature to be born, grow up and have their own babies. It's kind of like a life cycle that repeats itself over and over.