What documents have you looked at so far?
I would expect it to be detailed in the Reference Manual for your stm32.
Typically stm32 tamper is a low-power way of sensing when a pin state changes. Your external circuitry needs to detect when tampering occurs (e.g. opening the box - perhaps by way of a microswitch, or powering-up after removal of a screening-can - the can shorts the pin to ground).
In the event of tamper, the stm32 can clear a specific block of memory and/or record the time the tamper-event took place. This can happen even when the stm32 is in very-low-power modes such as sleep or stop. But some power must still be present - perhaps from a back-up battery, supercapacitor or coin cell.
It's then up to your main code (when normal power is restored if necessary) to check if tamper happened and do whatever you feel appropriate. This might be as simple as refusing to work until a special password is entered, or you might choose to erase the FLASH and brick the device.
