A showcase is a glass box where you can put things to show them off.
A showcase can make people think of museums, stores, or special events where people show off things.
Examples:
The jewelry store has a showcase full of diamonds.
I put my favorite toys in the showcase.
The showcase at the museum has old coins in it.
The showcase is so full, it might break!
In the showcase, there are magical things from a faraway land.
The showcase is so big, it can fit a whole car!