What do "cycles" in a capacitor mean?
It's your mains frequency (cycles per second). And MFD means microfarads, which you probably know already. You won't find a spec like that on the capacitor datasheet. 

That's really, really old. 25Hz has not been used widely in a very long while. Your mains frequency is either 50Hz or 60Hz, so 6uF is the value. Expect a modern replacement to be much more compact. 

Similarly the voltage needs to be rated at least as high as the original, but higher isn't a problem. 

You need to make sure you have the correct type of capacitor- if you put a polarized capacitor on an AC circuit you could have more excitement than you probably want. 

Given the age- the other parts may be paper capacitors- paper and foil wrapped together and dipped in wax. You can generally replace them with film capacitors.