In the modern world, every person alive is bound by the social contract in which they live. International law requires everyone to be within at least one nation's jurisdiction. Voting with your feet is the act of dissenting from your country's jurisdiction by moving to another. It can also mean vocally boycotting a certain company. .
Liberals expressed the idea of voting with their feet by moving to Canada, gaining citizenship, and renouncing their American citizenship if George W. Bush were to be re-elected, if John McCain were to win the Presidency, if Mitt Romney were to win the Presidency, and most recently, if Donald Trump will win the Presidency. Unfortunately, the only recent example of liberal cowardice to follow through with their threats is the formermost; however, in the likely event that Trump wins the Presidency and in the unlikely event that they follow through, liberals will soon discover the two-year minimum waiting period of gaining Canadian citizenship, and will once again be exposed as fools.
Edward Snowden did this in a way, by fleeing to Russia in order to escape justice for his crimes.
Unfortunately, liberals have "voted with their feet" to escape the high taxation and regulation of their former states, only to vote for those same failed policies in their new location. As a result, previously strong red states like Nevada and Colorado have turned purple or reliably blue.