The second term gives the number of feet in each line. The most common in English are pentameter (five beats per verse) and tetrameter (four beats per verse); other possibilities are monometer, dimeter, trimeter, and hexameter.
No meter is perfectly regular. Apart from the theoretical problem that no two syllables will receive precisely the same stress, most poets (even the most apparently regular) try to vary their verse by introducing occasional metrical substitutions.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.