The poem opens with the speaker telling the virgins to gather their ("ye") rosebuds while they still can ("while ye may"). "Old Time," after all, is passing quickly ("a-flying").
The "a" in "a-flying" doesn't really mean anything; it's just an older way of pronouncing a verb.
"Ye" is an old word for "your" and "you."
It's not clear if the speaker is referring to actual rosebuds, or if they are a metaphor for something else. We'll have to wait and see.