"Paul is dead" is a rumor that started in the 1960s that Paul McCartney, a member of a famous band called the Beatles, had died and was secretly replaced by a look-alike. The rumor says that Paul died in a car accident in 1966 and the remaining members of the band covered it up by putting a new person in his place.
But that's all made up! Paul McCartney is very much alive and has continued to make music for decades. The rumor started as a joke but many people believed it and spread it around, even looking for hidden clues in the band's music and album covers. However, the Beatles themselves have said the rumor is false and they're all still good friends.
So, you can listen to the Beatles' music and know that all four members were real people who worked together to make something amazing, and that Paul is still around to enjoy it all with us.