Redirecting standard output and standard error
The first specifies that all error messages should be sent to /dev/null
The second says to redirect standard output to /dev/null and redirect standard error to standard output, meaning all output to both standard output/error are redirected to /dev/null.
The third, 2>&1 indicates that the standard error (2>) is redirected to the same file descriptor that is pointed by standard output (&1).
2 represents standard error and 1 standard output