An attache case is a special type of suitcase that grown-ups use for carrying important papers and stuff. It's a hard-shell case, which means it's really strong and can't get squished easily.
When you open an attache case, you might see a bunch of pockets and compartments. This is because it's usually used for organizing and protecting things you don't want to get wrinkled or messy.
Attache cases are mostly used by people who work in offices, like lawyers or businessmen. They use it to bring important documents to meetings or to travel with their work stuff.
Sometimes, you might see people in movies carrying an attache case that has secret spy stuff inside. But in real life, it's mostly used for boring grown-up work stuff.