What is the meaning of MASKED by
The meaning is that the system doesn't see the corresponding IF flag. The IF flag is set when the event linked to the interrupt happens, but when IE is off, the system behaves as if the IF flag was cleared.
GIE is a global flag - it controls whether an interrupt routine happens or not. Even if GIE is off, the system may still use the IF flag - for example the IF flag can be used to wake the CPU from sleep regardless of the GIE setting.
The IE bit disables the IF flag and the system behaves as if the IF flag is zero. You, in your code, can still look at the IF flag to find out if the event has happened. This way you may be aware of the interrupt condition without enabling interrupts.
If both IE and GIE are set then the interrupt routine (ISR) gets called, and will be called to the rest of eternity until you clear the IF flag. In this case, if the non-ISR code reads the IF flag, it always be cleared, because if the IF was set the non-ISR code would never execute.
PEIE is just an additional flag which works as GIE, but only affects some of the interrupts.