The title of "To the Virgins, to Make Much of Time" lets the reader know what this poem is about right from the get-go. The speaker here has a clear audience, and a clear point: he's here to tell the young women of the world to use their time well. (Note that, at the time of the poem's writing, "virgins" was almost certainly meant as a reference to young women. For more on how the reader knows that these "virgins" are very likely women, and the speaker very likely a man, take a look at the Themes and Speaker sections of this guide.)
With that title, it's no surprise that the poem begins with a command: "Gather ye rose-buds while ye may." This apostrophe, pointed straight at the "virgins" of the title, lets the reader know that to "Make Much of Time" isn't a matter of, say, learning a new language or reading a good book or volunteering down at the soup kitchen.
The clue is in those "rose-buds." Roses are an ancient symbol of love, sex, and female beauty, and their early appearance here—plus the speaker's specific address to "virgins," women who haven't yet had sex—firmly places this poem in the realm of romance. There's also a general feeling of sensual pleasure here: think of the delicious smell of a new rose, and the warm summer weather that brings the flowers into bloom.
That the speaker encourages young women to gather "rose-buds," plural, also suggests that he's a bit of a libertine. He's suggesting that these ladies gather, not one perfect rose-bud, but whole bouquets of sexual experience.
Why? Because "Old Time is still a-flying." With the familiar personification of time as an old (but speedy) man, the speaker introduces the poem's major idea: life is short, and you've got to have fun while you can.