What is/are these serious repurcussions John Kerry speaks of?

A show down is brewing in the Ukraine as John Kerry has drawn a line in the sand saying

“There will be a response of some kind to the referendum itself,” Kerry said. “If there is no sign [from Russia] of any capacity to respond to this issue… there will be a very serious series of steps on Monday.”

I feel as though I need an interpreter to spell out just what Kerry meant by his words. Any Jellies here that can take a stab at explaining what a “very serious series of steps on Monday” means?? What exactly is Russia up against if they do not stop bullying Ukraine and what are we in the US specifically prepared to do to convince Russia they have to stop this nonsense and back off?