Orginally started off as a typo for "the", but recently has changed a bit in meaning. "Teh" still means "the", however, it is mostly used in the same manner that spanishspeaking people use the word "El".
Originally just a misspelling from people typing too fast, "teh" has become a word used by net.writers to exemplify people who are ignorant or simply not thinking clearly (perhaps because they're typing too fast).
I was reading a fundamentalist site the other day, and the writer was all upset about kids listening to rock'n'roll. He believes it's TEH DEVIL MUSIC FROM HELL.