It means you, as a reservist, could be involuntarily mobilized (not the unit, as is generally the case in the ARC, YOU) to do a 365 to the AOR or a ship. And it's not a rare or random thing - CNAFR gets a certain number of mobilizations per year. Some billets get volunteers, but others are involuntary and passed down to the units to pick candidates. Once you are picked, you cannot separate (IRR) or turn it down.