Ping is the time it takes for your computer and the server to send data between each other. Higher is obviously worse as you will be seeing the server several milliseconds after someone with a lower ping.
Fluct seems to be linked to loss.
Loss seems to make chat take forever to show up. Other ships will also slide very long distances if you have high loss.
Lag is how much you jump around for other people.
This "guide" is simply what I see happening when I experience different levels of these. Keep in mind I have no idea what is actually going on.
I have a tasty connection giving me a ping of 350ms and 0 fluct, loss and lag. So my connection gives others a slight edge due to higher ping. I can still tangle with the pvp enthusiasts and last a fair amount of time.
Mass fighter fights bring my loss up dramatically, meaning I see lots and lots of sliding ships that razor me without fail.
If someone is lagging around and hitting you perfectly, well tough. This ain't a pvp server so it's not like it matters that much (if it was, I would definitely bring up some sort of lag kicker).
Disco is for RP, complaining about a pvp problem that won't change is pointless.