The lyrics of the song are:

"What is a youth? Impetuous fire.
What is a maid? Ice and desire.
The world wags on.

A rose will bloom
It then will fade
So does a youth.
So do-o-o-oes the fairest maid.

Comes a time when one sweet smile
Has its season for a while...Then love's in love with me.
Some they think only to marry, Others will tease and tarry,
Mine is the very best parry. Cupid he rules us all.
Caper the cape, but sing me the song,
Death will come soon to hush us along.
Sweeter than honey and bitter as gall.
Love is a task and it never will pall.
Sweeter than honey...and bitter as gall
Cupid he rules us all."

The song essentially deals with the transitory nature of life. What the title specifically asks is, "What is it being young?" The answer provided in the lyrics clearly states that nothing endures forever. "Youth(fulness)" is therefore temporary. Throughout the song there are references to the temporal nature of things: roses bloom and eventually fade and so do young men and women - they age and then pass on. Sweetness has its time and then goes. The reference to death emphasises our impermanence.

The first few lines seem, however, to suggest a contrast between 'youth' and 'maid' where the one refers to a boy and the other a girl. The term, 'youth' had generally in the past been used to refer to the male gender. The modern use of the word nowadays refers to all young people, such as the main characters in the play. Be that as it may, the general idea is about young love. 

Love, in the form of Cupid "rules us all" and it "is a task" that will never "pall". The song provides both a dramatic and romanticised perspective of love. Love is a paradox, it can be both bitter and sweet. The song fits in well with what the play presents: that even a love as deep and passionate as that of our two 'star-crossed lovers' has an end, whether through tragic circumstance or choice. 

The song "What is a Youth," performed during the party scene in Romeo and Juliet, is about the radical extremes associated with youth, particularly youth who are in love. Lines like the following also foreshadow many events of the play:

"A rose will bloom
It then will fade
So does a youth."

Already, early on in the movie, the end is foreshadowed in this song. As previously mentioned, this song also deals with the idea of the impetuous and radical nature of youth who are in love. It talks about how misguided these hormanal youths are. This is illustrated in the following lines:

"What is a youth? Impetuous fire.
What is a maid? Ice and desire.
The world wags on."