what is the pause key on a keyboard
Break key On many modern PCs, Pause interrupts screen output by BIOS until another key is pressed. This is effective during boot in text mode and in a DOS box in Windows safe mode with 50 lines. On early keyboards without a Pause key (before the introduction of 101/102-key keyboards) the Pause function was assigned to Ctrl+NumLock, and the Break function to Ctrl+ScrLock; these key-combinations still work with most programs, even on modern PCs with modern keyboards. Pressing the dedicated Pause key on 101/102-key keyboards sends the same scancodes as pressing Ctrl, then NumLock, then releasing them in the reverse order would do; additionally, an E1hex prefix is sent, which enables 101/102-key-aware software to discern the two situations, while older software usually just ignores the prefix. The Pause key is different from all other keys in that it sends no scancodes at all on release; therefore it is not possible for any software to determine whether this key is being held down.[1]
AltGr key AltGr (also Alt Graph, or Right Alt[1]) is a modifier key found on some computer keyboards and is primarily used to type characters that are unusual for the locale of the keyboard layout, such as currency symbols and accented letters. On a typical, IBM-compatible PC keyboard, the AltGr key, when present, takes the place of the right-hand Alt key. In macOS, the Option key has functions similar to the AltGr key.