It's a recording trick known as backmasking, whereby words and phrases are reversed and inserted into songs, so that when played backwards the secret messages with hidden meanings can be heard. Backmasking was "exposed" by right-wing Christian groups in the '60s, '70s, and '80s as an attempt by major recording artists to convert unwitting people to the Devil, insisting that the reversed lyrics seeped into a person's subconscious and altered their minds (also known as subliminal messaging, which doesn't at all work like that).
Here are notable examples of music with backwards lyrics, most of which are purely imagined, though some are real and intentionally inserted into the songs.
Led Zeppelin - "Stairway to Heaven"
One of the most infamous examples of supposed backmasking. The message here is supposed to be, "So here's to my Sweet Satan / The other's little path / Would make me sad / Whose power is faith / He'll give those with him 666 / And all the evil fools / They know he made us suffer sadly." You can find numerous other interpretations of the same message, because of course the words themselves are not at all clear when you play the song backwards. I personally thought I heard Robert Plant say, "I wish it would snow." Read into that what you will.
The Eagles - "Hotel California"
Apparently, Don Henley can be hard saying, "Yes, Satan organized his own religion," or something to that effect. Another message "clearly" in the song is "Satan hears this, he had me believe," It always sounded like "Seattle Englishman nursery," rather than anything remotely Satanic.
Choking Victim - "Hate Yer State"
At the beginning of the song, there at first appears to be gibberish, but when reversed, a voice clearly says, "You think you're alive, motherfucker? You're just the walking fucking dead, you're a fucking sheep, stepping on my back to stay alive. West Coast, East Coast, you're all just a bunch of fucking fools, you and the rest of this greedy fucking world. Kill yourself! So stay in school, say no to drugs, oh yeah! Hail Satan! Good night boys and girls, pleasant dreams." Another intentional instance of backmasking.
Cradle of Filth - "Dinner at Deviant's Palace"
This song contains "The Lord's Prayer" in reverse (meaning, if you play the song in backwards, you will hear a voice reciting the prayer. This isn't so much a hidden Satanic message as a direct one: reciting "The Lord's Prayer" backwards is apparently a custom in some Satanic rituals.