I accuse you of not trying in any way to research or comprehend what it is. I dont expect any more then 5% effort out of a typical post and this is like 1.5.
While Ill cut you some slack becuase "hermie" in google didnt bring up anything on the FIRST PAGE "hermie pot" or "hermi weed" would have answered your question.
A hermie is short for hermaphrodite. What this means is that a male or female plant shows charicteristics of the oposite sex.
As growers are constantly in search of the glorious sensimilla we clone females or cull out the males in a crop to get this more potent weed. This leaves an all female group of plants that achive sexual maturity but because they recieve no male genetic material (pollen) they never spend any drug making energy on seed making.
Sometimes however, mostly do to enviromental stress, certain parts of even a female plant will develope male pollen sacks. This is when a plant "goes hermie" This can be really tough to spot and its the reason that your all female crop sometimes has seeds in it. Its definatly not as huge a deal as some think, I didnt catch a hermie in my last crop and got a couple of seeds out of it, but it was so few they were like a bonus.
Common causes are light leaks during flowering darkness. Thats what i get paranoid about.