Software often breaks when moved between machines because the surrounding libraries and settings differ. A container bundles the application together with the exact dependencies it expects, so it behaves the same on a laptop, a test server and production.
Containers are lighter than full virtual machines because they share the host's core system rather than each carrying its own. That makes them quick to start and easy to run in large numbers, which is why they underpin much of how modern services are deployed and scaled.