Answer: must understand that fighting for freedom is extremely difficult.
In this excerpt, Paine argues that people who really desire freedom (in this case the colonists) must understand that it is extremely difficult to obtain it. In order to "reap the blessings of freedom," these men must "undergo the fatigues (efforts and suffering) of supporting it." Only if we are willing to fight and to make sacrifices for it will we be able to enjoy it.