Back when times were good and love was easy, the lord and lady of "The Husband's Message" spoke vows to one another. Now a messenger has traveled across the sea to remind the lady of those vows and her obligation to reunite with her lord. To convince her to do so, the messenger assures her that the lord has no intention of breaking his vows.
A vow or an oath – a promise, spoken aloud – is taken super seriously in Anglo-Saxon poetry. Your oath is your whole integrity as a person. A vow or oath normally requires some sort of action on the part of its speaker, which is the situation of the lady here. Yet what's unique about this vow is that it's a promise made between two people, which might become invalid if one of the parties breaks it. That's the reason the lord feels he has to assure the lady of his continued commitment to their vow. And not only that, he sends runes to "speak" his oath of vow-upholding for him. By leaving the lady in no doubt of the continued validity of their oath, he hopes to make her feel duty-bound to reunite with him.
The lord must convince the lady of his continued commitment to their vows because they are a covenant, which would be invalidated if one party failed to uphold it.
The runes at the end of the poem enact in writing what lovers' vows do in reality.