Well, I'm sure Jhan will explain it better, but I'll try to explain what I think it means.
Let's say I'm following a good diet and supplements and other methods to cleanse my system of all the junk that's built up in it over the years. The cleansing causes die-off, which means the toxins that have sat stored in my body are now released into my system to be washed out through the normal channels (skin/bowels/bladder). Toxins released into my system make me feel bad, because my organs have to experience their passage and their effects(die-off symptoms). While floating around inside me, making their way to their final exit, they come across the Candida I'm also trying to flush out. Candida loves to nourish itself on toxins. So, while it and the toxins are heading south, it gorges itself on them and increases it's presence (kind of like how I increased MY presence by an extra 10lbs. after my honeymoon cruise several years ago...but, that's another story), which also makes me feel bad because I've got a sudden upswing in my Candida count. But soon the Candida and the toxins come out of me with all the pomp and ceremony that frothy urine can provide (assuming I didn't do anything else to create the froth, such as stop by Krispy Kreme on my way home from the health food store ;o) ). I can just imagine the foamy little critters looking back up at me from inside the bowl, flipping me the bird as they circle and spiral toward their next destination. So long as they're out of me, I don't care what parting shots they'd have to share. ;o)
The difference between this kind of Candida increase and the kind that is stimulated by not maintaining good health practices is that this increase is brought on by the process of detoxing my body, and is somewhat temporary. As I continue to detox my system over time, there is less and less for the Candida to nourish itself with, thus effectively starving it to death. I am creating an inhospitable environment for it's survival.
You'll often hear people say they feel worse for a time or now and again on their new diet and supplement regimen. They may worry that they are getting worse from it, and consider or actually (cringe) stop doing the good they had started. In reality, again assuming they are not cheating excessively, they are most likely experiencing a positive yet not always comfortable healing. Kind of like cutting open a wound to get the puss out of an infected injury, it can often hurt much worse at first, or at the very least not feel any better. But, as the puss (in this case Candida/toxins/bacteria/parasites/name your affliction) slowly drains, the body is given the chance to heal, and the wound over time settles and clears away.
Anyway, that's my take on what's likely happening with me when I get a little extra froth in my urine. I also measure how I feel, what I call my core or base energy, and if I'm feeling stronger deep down, then I suspect the symptoms I'm having on the surface are probably die-off and in the long run a good thing. Because as time goes by, and there's less and less toxicity in my body, there will be less and less renegade Candida making their way out of the "colon corral", running rampant through my system. I mean, what's the point of them leaving the colon if there's nothing fun for them to do outside of it??