William Wordsworth's "Lucy Gray" contains the following meter: alternating lines of iambic tetrameter and iambic trimeter. 

An iamb (or iambic when placed with the number of feet) is a singular metric foot containing an unstressed syllable and then a stressed syllable. Tetrameter refers to the existence of four poetic feet (iambs), and trimeter refers to three poetic feet (also iambs). 

A tetrameter "looks" like this (u=unstressed and /= stressed): u/ u/ u/ u/

A trimeter "looks" like this: u/ u/ u/

Here are the first two lines of the poem marked for stress.

Oft I had heard of Lucy Gray,

u   /   u      /     u   / u     /

And when I cross'd the Wild,

  u      /   u     /       u     / 

Here it is again with non-bolded letters signalling the unstressed words and bolded letters signalling the stressed syllables:

Oft I had heard of Lucy Gray,
And when I cross'd the Wild,

One can see how four pairs of feet exist in the first line:  "Oft I," "had heard," "of Lu," cy Gray."

Three exist within the second: "and when," "I cross'd," "the wild." 

This alternation of iambic tetrameter and iambic trimeter extends throughout the poem.