Sustainable capitalism means that we can still have a healthy environment and stable economy while we make and sell things. It's like playing with blocks - we want to build a tower that doesn't fall down, but we also want to use the blocks in a way that makes sense and doesn't waste any of them.
In the same way, sustainable capitalism means that we want to make sure we're using resources in a way that doesn't harm the environment, but still allows us to make and sell things that people want. We want to use resources efficiently, so we don't waste any materials, and we want to make things that last a long time, so we don't have to keep making more and more of them.
This helps us keep the Earth healthy for a long time into the future, so that we can keep playing with our blocks and building things for years to come.