Oxford Dictionary defines this idiom as follows:
You are in a position to take the opportunities that life has to offer.*
Her son is in a brand-new city for college. There are so many opportunities and so much to explore. The world is his oyster!
Like many commonly used American idioms, “the world is your oyster” came, not only from Old English (like most), but it was first used by author William Shakespeare!
It can be seen in his “The Merry Wives of Windsor.”
Falstaff: I will not lend thee a penny.
Pistol: Why then the world’s mine oyster, Which I with sword will open.
Falstaff: Not a penny.**
While this example has somewhat violent, and perhaps less than ethical implications, it is currently used to mean that the world is ours to enjoy and encourages us to take advantage of every opportunity we can.