It 's an idea you have about the future, which is unlikely to come true. Getting obsessed about an unrealistic expectation, such as eating too much fruit and vegetables, might make you frustrated when you are not able to fulfill that expectation. I hope I 've explained myself well enough :)

higher expectation also "impossibile"

或者用中文来讲就是人体每天对水果蔬菜的吸收是有限的，如果超过限度就是不现实的期望。“unrealistic expectation”