>/dev/null redirects the command standard output to the null device, which is a special device which discards the information written to it
2>&1 redirects the standard error stream to the standard output stream (stderr = 2, stdout = 1). Note that this takes the standard error stream and points it to same location as standard output at that moment. This is the reason for the order
>/some/where 2>&1 because one needs to first point stdout to somewhere and then point stderr to the same location if one wants to combine both streams in the end.
In practice it prevents any output from the command (both stdout and stderr) from being displayed. It's used when you don't care about the command output.