Like "Mary Jane" but obviously, less conspicuous to higher authorities.
Named after a big pothead from Sacramento, "Katie Drew" became a pseudonym of sorts for marijuana. You can use "Katie Drew" in place of the word "weed" or any other obvious slang word used for marijuana in front of parents/teachers/peers.
The phrase "going to Katie's house" is also useful when talking about going out to get stoned.
Girl: Yo, do you like Katie Drew?
Boy: Nah, not into her.
Girl: We gonna go to Katie house after school?
Boy: SO down.