I got a 100 watt CWA mercury vapor ballast for Christmas and it says "10 microfarad 240 volt cap" (which are readily available as fan capacitors) but it also says "2.60 MEGOHM MAX Rdis". What does this mean and will it affect performance of the ballast?

I'm guessing resistor discharge since a lot of HV capacitors have built-in or paralleled discharge resisters ranging from less than 500KΩ to 2MΩ or more. So it's likely saying not to use a capacitor with a self-discharge resistor over 2.6MΩ.

Thank you! That sounds very plausible!

It is for safety reasons: To make sure when the power is disconnected, there is no unsafe voltage (so the capacitor discharged) after a given time limit. The time depends on the voltage the capacitor could be exposed and the time constant of the capacitoance and the discharge (aka bleeder) resistor.

Usually for a PFC capacitor across the supply as we have here, it is to ensure there is no unsafe voltage (≯50V) after approximately 1 minute.

So if I were to use it without the discharge resistor, it wouldn't damage the ballast at all, but I could get shocked?

So if I were to use it without the discharge resistor, it wouldn't damage the ballast at all, but I could get shocked?

Only if you touch both the capacitor terminals