You've seen it advertised before: this [insert name of watch here] is shock resistant. But what does that really mean?
According to the U.S. government, a watch is considered shock resistant if it can withstand an impact equal to that of being dropped onto a wood floor from a height of 3 feet.
In practice, however, shock resistance is usually tested by applying two shocks (one on the 9 o'clock side, and one to the crystal and perpendicular to the face). The shock is delivered by a hard plastic hammer mounted via a pendulum. To pass the "hammer test," the watch must keep its accuracy to +/- 60 seconds/day as measured before the test.
This is especially an important concern with mechanical watches as broken balance wheels were a common cause of watch repairs before the the widespread adoption of shock resistant balance pivots in the 1950s.