It’s called backmasking. It’s a recording technique where message is recorded backward onto a track that is meant to be played forward. It was popularised by The Beatles, who used backward instrumentation on their 1966 album Revolver.

In 1969, DJ Russ Gibb received a phone call from a student at Eastern Michigan University who asked Gibb about a rumor that Paul McCartney had died. He claimed the song Revolution 9 contained a backward message confirming the rumor. Gibb played the song backwards on his turntable on the air, and heard,

“Turn me on, dead man … turn me on, dead man … turn me on, dead man …“

Gibb began telling his listeners about what he called “The Great Cover-up”, and added various others, including the alleged backmasked message in I’m So Tired,

“Paul is a dead man, miss him, miss him, miss him.”

Some Beatles songs backmastered here are,

Sgt. Pepper’s Lonely Hearts Club Band
Strawberry Fields Forever
All Together Now
Blue Jay Way
I’m so tired
Revolution No.20
Revolution No.9
Long Long Long
Let It Be

And there’s Kiss Kiss Kiss– from John & Yoko’s Double Fantasy.

Listen.