"let's roll" has been used extensively as a term to move and start an activity, attack, mission or project. The phrase has been widely appropriated as battle cry. After the attack of September 11, 2001 the phrase , mainly in the United states symbolize heroism and self sacrifice. The phrase may have origin as early as 1908.
I think that all of these responses are correct. Ultimately, it means "let's go." Although the normal, daily use might mean "get in the car and go," "let's roll" also has a military meaning. To me, it has a more determined feeling than "let's go." In other words, "let's move no matter what the circumstances." It might also be a shortened "let's rock and roll," which also has a military and more aggressive/confident connotation (like its musical origin).