Don't worry about the complicated name for this poem's meter; it sounds worse than it really is. "To the Virgins" alternates between two different types of meter. The odd-numbered lines (1, 3, 5, etc.) are all in iambic tetrameter. That means there are four ("tetra") iambs per line. An iamb is a beat that contains an unstressed syllable followed by a stressed syllable. It makes a sounds like da-DUM. Take the third line of the poem as an example:
And this | same flower | that smiles | to-day
(Hint: "flower" is often read as a single syllable in English metrics.)
The even-numbered lines (2, 4, 6, etc.) are all in iambic trimeter with catalexis. That means there are three ("tri") iambs plus one extra syllable. (Catalexis comes from the ancient Greek word for "leave off"). Basically, it's almost like the tetrameter line, just a little bit short, since it "leaves off" the last stressed syllable. Here's an example from line 6:
The high|-er he's | a-gett|-ing
(Hint: the "-ing" is the catalectic element.)
There are definitely variations on these meters. Not all the lines fit perfectly. For example, the first foot (i.e., beat) of the first line is not an iamb (da-DUM) but rather a trochee (DUM-da, a reverse iamb), and he rest of the line consists of three iambs:
Gath-er | ye rose|-buds while | ye may.
Good news: the rhyme scheme is much less complicated than the meter. It's actually just a simple ABAB rhyme scheme, which means that the last words of alternating lines rhyme. Here's an example from the first stanza:
Gather ye rosebuds while ye may, (A)
Old time is still a-flying: (B)
And this same flower that smiles to-day (A)
To-morrow will be dying. (B)
Phew! Nice and easy.