It means he has made a proposal (asked her marry him) but it has not been accepted yet (She has yet to say yes)

Monday: "He is going to present his proposal tomorrow." Wednesday: "He presented his proposal yesterday. It was well received, but it is yet to be accepted." Friday: "His proposal was accepted."

To add to Stevii's answer, "is yet" or "has yet" or "as of yet" or "yet" usually implies expectation. It could also further imply that one is actively waiting or worrying on that expectation. So, he has proposed something to someone, perhaps a marriage proposal or business proposal, and he is expecting an agreement (or a "yes" answer to a marriage proposal). Contrast this with "His proposal has yet to be answered." (or "is yet" -- but I'd use "has yet" in most cases, including your example) In this example, the person is awaiting or expecting an answer. We assume he is hoping for acceptance, but it doesn't imply that he's expecting the acceptance, as your example does. Though he is expecting some kind of answer either way. Just to illustrate a little more how "yet" affects meaning: Q: "Did you find your umbrella?" A1: "I haven't found it." A2: "I haven't found it yet." In A1, the person hasn't found it. She (or he) may have stopped looking and may have now come to the conclusion that she has lost her umbrella. It would depend on her tone when she says it. However, in A2, the person is using "yet" to signal that she expects to find it. She is implying that she hasn't given up looking and is still looking for it (or at least telling herself that she's going to look for it again in the future) and still expects to find it. Q2: "Have you found your umbrella yet?" A3: "I haven't found it." (or "No." or "No, I haven't." or "No, I haven't found it.") A4: "No, and I don't think I'm going to." In Q2, the person is using "yet" to imply that the other person is still looking for her umbrella. In answer A3, the answer is the same as A1 but in this case the person asking used "yet", so the answer actually implies the same as A2 above, that she will probably continue to look for it. It is a little ambiguous however. So she may use A4 to explicitly state that she has given up the expectation of finding it.

It means that the proposal has been made already, but not accepted (or agreed to).