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 spanish speaking people use the word "El".
"I got two cookies with teh whipped cream!" makes sense, but "I got two cookies with the whipped cream!" doesn't. Get what I mean?
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.