Green’s Dictionary of Slang

1. (orig. US) to be aware, culturally sophisticated; thus 1960s hippie n.² (3) slogan ‘turn on, tune in, drop out’.

1923		N. Anderson Hobo 4: The radical, the optimist, the crook, the inebriate, all find others here to tune in with them.
1968		L. Wolf Voices from the Love Generation 88: When I first heard Leary say, ‘Turn on, Tune in, Drop out,’ it really upset me [...] ‘Tune in’ takes care of doing things that are meaningful and developing your own mind.
1999		Guardian Media 2 Aug. 3: The Harvard psychology lecturer’s slogan ‘Turn on, tune in, drop out!’, became a central dogma for hippies.

2. to make someone aware, to explain.

1958

Jimmy Logsdon ‘I Got a Rocket in My Pocket’ 🎵 Better tune me in and get my signal right / Or there’ll be no rockin’ tomorrow night.