What is the mean of process corner?

Voltage and temperature are only two of the factors that determine the actual speed of a device. "Process" is another.

You have some control of the voltage and temperature, but you have no control of the process. When an IC is manufactured there are variations in the process. For example, exactly how thick a layer gets deposited on the wafer. The machines that do this are not perfect, so there is always some variation. It will vary "lot to lot", as the machine that applies the layers ages, or perhaps it is effected by the temperature at the time. It will vary wafer to wafer, as the environment may change over a short period of time and that simply the application is never exactly the same twice. It will also vary within a wafer. A wafer may contain a large number of the actual "die" that makes up the FPGA. The die that were located in the center of the wafer will be different from the ones out at the edge of the wafer.

In the ASIC manufacturing world, there are MANY "process" variables. There could easily be 80 different layers applied, each one could have a variation. When the manufacturer of the part proves to themselves that the part works they have to consider a lot, if not all, of the permutations. That is way too much work to consider including in each of our designs, so they will typically take the two extremes: Fast and Slow.

There is a tolerance that is allowed, but the parts you get should meet the minimum requirements. It could also be a lot faster. In many cases, the parts are sorted by the manufacturer and faster parts sold for more money.

So, your "graph" needs to be three dimensional: Voltage, temperature, and process.

Rod