The context of the lines you ask about is the quoted passage added above. The specific reference you ask about is one of Shakespeare's puns with sexual innuendo.
- pun: a play on words that gives a different senses to the same word
- innuendo: a subtle or subtle expression that implies of suggests rather than states outright
In this pun, Shakespeare is punning off the word "horns" giving it a non-literal, figurative meaning. In normal language usage, horns are the projections on the heads of some animals, like cows and goats ... and snails. In Shakespearean innuendo, horns are the mystical ornamentation that men whose wives have strayed in faithfulness are said to be plagued with. In other words, if a husband is unfortunate enough to have a wife who strays in her attentions, he is said to be a "cuckold" who has been "cuckolded." Cuckolds are to be recognized by the mystically appearing horns upon their heads.
What Rosalind is saying in this otherwise nonsensical exchange of low humor is that Orlando is late for their meeting: she would rather be courted by a snail than by someone who breaks love's promise by being late. Orlando wonders why a snail would be better. Rosalind says it is because (1) he carries his house with him, thus (in punish theory) his wife is always with him and because (2) he already wears horns so no one can ever accuse his wife of secret infidelity (or him of being a cuckold) by the sudden appearance of horns on his head (horns cannot suddenly appear because he already has them, and they have nothing to do with his wife).
The conclusion of this ribald word play is that Orlando declares that a wife's virtue will prevent any husband from being cuckolded and horned and that Rosalind is virtuous. In sum: "horns" are the mystical result of a cuckolded husband after his wife's infidelity. Ganymede/Rosalind would prefer to be courted by a snail than by a late suitor who breaks love's promises because, being horned already, a snail can never slander a wife (no matter what she does, the horns can never reveal her behavior). To this Orlando replies that virtue prevents horns, and Rosalind is virtuous. Seemingly, this entire pun is for the purpose of characterizing Rosalind as virtuous so we know in what light to understand what is to follow between Rosalind and Orlando in their role play.
Virtue is no horn-maker; and my Rosalind is virtuous.
Come, woo me, woo me, for now I am in a holiday
humour and like enough to consent. What would you
say to me now, an I were your very very Rosalind?
I would kiss before I spoke.
Nay, you were better speak first, ... for lovers lacking ... [talking] matter, the cleanliest shift is to kiss.