tail-chaining of Interrupts

Tail-chaining is back-to-back processing of exceptions without the overhead of state saving and restoration between interrupts. The processor skips the pop of eight registers and push of eight registers when exiting one ISR and entering another because this has no effect on the stack contents. Cortex™-M3 Technical Reference Manual

Which basically means, handling pending interrupts without repeating the stacking.

I recommend this book if you want to know more details:

The Definitive Guide to the ARM Cortex-M3

If an exception is in pending state when another exception handler has been completed, instead of returning to the interrupted program and then entering the exception sequence again, a tail-chain scenario will occur, where the processor will not have to restore all register values from the stack and push them back to the stack again. The tail-chaining of exceptions allows lower exception processing overhead and better energy efficiency.