IR Drop

What is IR Drop?

V = I x R is the Ohm's law.
Basiclly everything that has current going through it also has an associated voltage drop.

IR Drop affects timing because the cells will not get sufficient voltage and thus cannot rise to the desired level within the rise time.

Essentially, your power straps are not with zero resistance.  So as your cells switch and draw current, there is drop in the VDD seen by the cells. This changes the cell characteristics like drive resistance and slow down the cell, causing timing violations.