MagSafe is a magnetic system used in the iPhone since iPhone 12 that allows you to connect accessories to the back of the phone. Comprising a ring of magnets embedded in the back of the iPhone, it allows you to attach to wallets, wireless chargers, grips and more.
Cases that say they support MagSafe accessories should have a built-in ring of MagSafe magnets, but some cases are advertised “MagSafe compatible” despite not having magnets – they're simply thin enough to allow the magnetic connection to pass through them. The problem is that this connection is usually quite weak and you may find it's very easy to separate your phone from the accessory.