A place to test
A payment provider such as Stripe may offer a test environment called a sandbox. Teams can try payments with test accounts and data, without moving real money.
A limit on what software can do
A phone may run each app in a sandbox that limits access to other apps and files. This reduces harm if an app misbehaves.
Both kinds reduce risk but cannot guarantee safety. A test system may differ from the live service, and app restrictions can have weaknesses.