Q:

What does "process" mean in PVT? 

For timing analysers, FPGA operating conditions are sometimes know as "PVT", which stands for "Process, Voltage and Temperature". 
While voltage and temperature are self explanatory, what does process mean in the context of FPGA operating conditions? 

A:

"Process" in this case refers to the manufacturing process at the plant where they make the FPGA.  It's a measure of the statistical variability of the physical characteristics from chip-to-chip as they come off the line. 
This includes everything from mask alignment to etching times to doping levels.  These things affect electrical parameters such as sheet and contact resistance, actual transistor gains and thresholds and parasitic capacitances. 
These kinds of variations are unavoidable, and the P in PVT is an attempt to account for their effects in the timing analysis.  The idea is to make the analysis conservative enough so that your design will work reliably despite these variations.