What does the "Maximum Frequency" number mean in the Windows Resource Monitor?

It simply means of the total maximum of your processors normal speed.

With speed step, power saving and everything else disabled, this should always read 100%.

If you have power saving on your laptop that under clocks your CPU compared to the stock speed, it will report a lower percentage.

If you have turbo boost or similar, it will report a higher percentage.

So, again, this is the current maximum percentage your processor can currently run when compared against its reported normal speed.

I am not 100% sure, but my guess is that if you overclock, the overclocked amount would be the "base" speed to Windows and overclocking by 20% would not show a 120% maximum frequency - this is just guessing, I have no way to test.

According to an answer here:

Maximum Frequency in Resource Monitor is the same as the Processor Performance \ % of Maximum Frequency counter in Performance Monitor.

For example if you have a 2.5 ghz processor which is running at 800 mhz then % of Maximum Frequency = 800/2500 = 31%. So the processor is running at 31%, or 800 mhz, of the processor's maximum frequency of 2500 mhz (2.5 ghz).

The "best" percentage of maximum frequency is subjective. Basically, you want the CPU running at a frequency that is fast enough to do what you want while using the least amount of power so it doesn't drain your battery or increase your electric bill unnecessarily.

Your power plan in Windows is part of what determines the frequency as well as settings in the computer's BIOS.

Take a look at the section Processor power management (PPM) may cause CPU utilization to appear artificially high in this article: Interpreting CPU Utilization for Performance Analysis

Very late reply, but I just noticed that my percentage in Resource Monitor for CPU frequency is 129%, which corresponds with my overclock. I have a 3.4 GHz Intel i5 that is overclocked to 4.4, which is a (1000/3400) * 100 = 29.411% increase over stock speed. Turbo Boost for my processor (the factory boost to frequency) was 3.8 GHz, but this also showed above 100%. Basically, the frequency your processor is listed at on the box and in CPU-Z at its maximum stock frequency (without Turbo Boost) is what Resource Monitor takes to be 100%.

Just to add that on a modern multi-core computer where unused cores are parked (turned off) to conserve power, the displayed percentage can be much less than 100%.

I have seen values of 30-50% on an unencumbered 8-core computer.