It means redirect file descriptor 2 to the same destination as file descriptor 1.

Which actually means that an undelrying dup2 operation happens in this direction:

   2 <- 1   // dup2(2, 1)

The file description at [1] is duplicated into [2], thereby [2] points to the same object. Anything written to stderr goes to the same device that stdout is sending to.

The notation follows I/O redirections: cmd > file actually means that a descriptor [n] is first created for the open file, and then that descriptor's decription is duplicated into [1]:

   n <- open("file", O_RDONLY)
   1 <- n