Hey there!
When we say nonrenewable, we mean that we use it today or it's able to be used today, but once it's gone, it's gone forever or it takes a long time to come back.
An example of this is oil.
When we dig for oil, we use lots of resources and when we find it, we use it at an ever-increasing rate. Think about it- we use it for cars, trucks, all vehicles, electricity, plastics, asphalt, and so much more.
However, oil just doesn't come and go as it pleases.
Oil can take millions of years to form- much like rare materials like diamonds. When we use it at this current rate, it can go very fast, and we'll have to find a good enough material to replace it- because it's not coming back anytime soon.
Hope this helps!