262 That leads your Muse in haughty verse to mask,
Muse > (The nine Muses are the daughters of Jupiter and Mnemosyne
(Memory); each presides over an area of the arts and sciences and
gives inspiration to its practitioners. The Muse referred to is
probably Clio, the Muse of History)
haughty > noble, high-minded
mask > disguise [herself]
263 and loath the lays that long to lowly swains.
lays > songs long to > befit, beseem; are appropriate to swains > young men
264 That lifts your notes from shepherds to kings, 265 So like the lively lark that, mounting, sings. 266 267 Your lovely Rosalind seems now forlorn,
Rosalind > (Colin Clout's love in SC)
268 and all your gentle flocks forgotten quite, 269 Your changed heart now holds your pipes in scorn, 270 those pretty pipes that did your mates delight. 271 Those trusty mates, that loved you so well, 272 Whom you gave mirth: as they gave you the bell.
the bell > the prize; first place (before cups were awarded to winners of horse-races, etc., a little gold or silver bell was presented as the prize)
273 274 Yet as you erst with your sweet roundelays
erst > previously, at first roundelays > {Short, simple songs}