Imagine you and your friends are playing with some toys at the playground. Some of the toys belong to you, but one of your friends brought a really cool frisbee that everyone wants to play with. Your friend lets everyone use the frisbee, but they still own it and get to decide when they want to take it back home.
The public trust doctrine is kind of like the rules that would apply to the frisbee situation if it belonged to everyone in the community instead of just one person. It says that certain resources, like water, wildlife, and public land, belong to everyone in the community, and the government has a responsibility to protect and manage these resources for the benefit of everyone.
Just like your friend had a responsibility to make sure the frisbee didn't get broken or lost, the government has a responsibility to make sure that the resources belonging to the public trust are used wisely and kept in good shape so that future generations can enjoy them too.
The public trust doctrine also means that individuals and corporations can't just come in and take over these resources for their own profit, because they belong to everyone, not just one person or group. This helps keep things fair and ensures that everyone has access to these important resources.