This poem, made even more modernly famous by the film "Dead Poet's Society," essentially speaks the overarching message:Carpe Diem, or "seize the day."
The most obvious symbol in this poem is the rosebud, and it holds many complimentary meanings. First, rosebuds represent youth and beauty. Because they bloom in spring, they represent new life, and because they die within one season, they represent a short life. Gathering rosebuds "while ye may" suggests that the time frame for seizing an opportunity is short.
But in this poem, with the word "virgins" in the title, rosebuds are clearly also a sexual symbol. Not only do they represent life, but they represent love and physical sensuality. Though the deeper meaning here is to seize opportunity while you are young, the speaker is clearly appealing to, and drawing comparisons to, the power of sexuality and sexual urge in young people. The fact is, such urges are strongest for a very limited time in the spectrum of life. The rosebud here presents both an innocent but also a very sexual image. If the reader were to seize opportunity with the same desire that he seizes sexual pleasure (or could), the message is that he will not be disappointed before he dies.