What are flops in VLSI?

Could you elaborate on what flops are in the context of VLSI design? I'm curious to understand how they play a role in the functionality and performance of integrated circuits. Are flops related to the timing and stability of signals within a chip, or do they have a different significance? I'm particularly interested in how designers incorporate flops into their designs to optimize power consumption and speed. Additionally, are there any common challenges or considerations when working with flops in VLSI design?