This quote is said by Rosalind in Act I scene 3, and comes as part of her speech when she plans to disguise herself as a man as she and her cousin, Celia, talk about their escape from court to the Forest of Arden to look for Rosalind's father. Rosalind is afraid of what could happen to them, as two young, beautiful maidens alone, and in particular is wary of the possibility of being raped. To protect them, she feels disguising herself as a man would present the image at least of male bravery, even if it covers over female fear. Note what she says:
A gallant curtal-axe upon my thigh,
A boar-spear in my hand, and in my heart,
Lie there what hidden woman's fear there will,
We'll have a swashing and a martial outside,
As many other mannish cowards have
That do outface it with their semblances.
Rosalind thus draws a parallel to herself an to other real men, who appear to be big, strong and brave on the outside, but are actually cowards on the inside. By adopting a "swashing and martial outside," Rosalind will, like these "mannish cowards," be able to present outwardly the appearance of a warlike and fearsome individual which will hide her fear and lack of bravery on the inside. Note how this quote ties in to the ubiquitous theme of appearances vs. reality, as Rosalind deliberately plans to disguise herself as something she is not so others will think she is not what she is.
The words "We'll have a swashing time...semblances" mean though she, being a woman, will be afraid at heart, she will put on a brave front. She will assume the swaggering airs and tricks of a soldier like the many cowardly men who put on a bold pretence and disguise the fear that they really fear. In this way, by their appearence and bravery they deceive their beholders who accept them as fearless. In this passage, they hypocrisy of male cowards is referred to. Rosalind's excuse is that she ia a female coward, so she has a better right to conceal the fears lurking in her by putting on a bold soldier-like outward appearence.