This slang expression gained widespread currency in the sixties as a hip way of stating that the speaker understood the essential truth of a situation: “I know where it’s at.” Or more commonly: “You don’t know where it’s at.” It is still heard from time to time with that meaning, but the user risks being labeled as a quaint old Boomer. However, standard usage never accepted the literal sense of the phrase. Don’t say, “I put my purse down and now I don’t know where it’s at” unless you want to be regarded as uneducated. “Where it is” will do fine; the “at” is redundant.