If you look up the word "frontier" in the dictionary, you're probably going to find a reference to a boundary or a limit. It can be an international border. Or it can be the limit of civilization, as it is frequently used in the context of American history. Regardless of the specifics, it is the end of something, the place where the world you know meets the unknown.
But beyond that threshold is also opportunity. That was the reality for tens of thousands in the middle of the nineteenth century, as every child who ever played Oregon Trail on classroom computers knows well. It does not come without risk, as became apparent to those same children while passing digital headstones mourning the loss of the beloved Stinky to dysentery. However, the hope of wealth and bounty trumped, for many, the potential costs of leaving behind their stable existence in the east. That is why so many went to Oregon or California in the 1840s and 50s. Or the Klondike in 1897 and 1898. Or Nome in 1900.