Metre is important when a musical passage has a text. However it can be relevant in other music. Metre refers to stresses on notes or words in passages of text or music.
The stress falls on the second syllable.
_ I (the _ refers to a un-stressed and I refers to the stressed syllable)
Iambic metres are flowing and allow for lyrical patterns. They could be used in "love" songs, and in impressionist music. Any music that has a flowing lyrical feel could have an iambic metre.
The stress falls on the first syllable.
Trochaic metres are disjointed and jarring in their feel. When passages are conflicting they may use a trochaic metre. For example in the set work "Dido and Aeneas" (Next year Mozart Jupiter Suite), the fight between Dido and Aeneas has a trochaic metre when she refers to Aeneas as a "deceitful crocodile".
The extension of these two metres is Anapaestic _ _ I, and Dactylic, I _ _