Not diversified. No optons.  Focusing all efforts, hopes or money on one opportunity. The problem with all eggs in one basket is, if you drop the basket all are lost; or if that basket is one stock exchange company and it fails, you lose all investment, so the phrase proposes diversification to hedge against disaster.(Compare to All in or Plan B.)

    “To enter college, he did not apply to several schools but put all his eggs in one basket by applying only to Harvard.”