When Abigail Adams writes this to her husband, she is quoting part of a line from Alexander Pope. Here is a bit more of the line she is quoting:
She, who ne'er answers till a Husband cools,
262 Or, if she rules him, never shows she rules;
263 Charms by accepting, by submitting sways,
264 Yet has her humour most, when she obeys;
To me, what Adams means here is that women are supposed to (and are able to) get their way by doing what they are told. They can "charm" the men by accepting (or seeming to accept) what the men say. They can "sway" them (persuade them) by appearing to submit (to give in). She is saying that women do have ways of getting what they want even when men try to keep them down. Before she quotes Pope, she writes
we have it in our power, not only to free ourselves, but to subdue our masters, and, with out violence, throw both your natural and legal authority at our feet ;
I think that what she means, then, is that men should give women the vote and give them equal rights in part because women already have power that they an use.