real time clock

Since the RTC is using a 32768Hz crystal as timebase, the RTX needs to count to 32768 before incrementing a second. That's the job of the prescalers. In calendar mode, the prescaler functionality is fixed and you can ignore their existence. In counte rmode, you can program the prescalers. The users guide explanation should be exhaustive.

lokesh bathala said:

help in using other interrupts while rtc interrupt.

you shouldn't try (or need9 to handle any other interrutps while already handling an interrupt. Interrupt functions are fast-in fast-out. If oyu need to do any lengthy work when an interrupt is triggered, just set  aflag that the interrupt happened and handle it inside main.

Allowign other interrupts isnide an ISR almost always leads to crashes and malfunctions that are next to untraceable. And it is almost never really necessary.

Why do you want to use RTC to get the interval of few seconds? you can easily use the timer for the same.

Use RTC only when you need to use date and time, otherwise I dont see any better use of RTC time/alarm compared to timers.