Final answer:
Frederick Jackson Turner believed that Western expansion was the American 'safety valve,' which provided an alternative to wage labor and helped alleviate tensions in the East caused by the scarcity of farmland and jobs. This belief was important as it shaped the idea that American democracy was formed by the frontier and contributed to the nation's push for foreign expansion.
Step-by-step explanation:
Frederick Jackson Turner believed that the American 'safety valve' was Western expansion. He argued that this expansion provided a release valve for tensions in the East by offering an alternative to wage labor and providing new opportunities for farmland and jobs. Without this 'safety valve,' tensions would increase as resources became scarce. Turner's belief in the importance of the 'safety valve' contributed to the idea that American democracy was shaped by the frontier and fueled the nation's move toward foreign expansion.