In poetry, stanzas are visual groupings of lines. A group of two lines is called a couplet. A three line stanza is called a tercet. A four line stanza is a quatrain, and a five line stanza is a quintet. Two other common lengths are a sestet, six lines; and an octave, eight lines.
For instance, you might break a fourteen line poem into three quatrains and a couplet, or into an octave and a sestet. Not only would the resulting poems look different on the page, their different looks would probably be reflected in the ways they develop their themes, and the ways they use sound to complement meaning.
If you know in advance that you want to write your poem in tercets, you can simply begin writing and thinking in three line units. Or you might write a twelve line poem with no stanza breaks and later divide it into four tercets, or three quatrains, or six couplets. Having done so, you could go back and make some revisions to adjust the original content to the new form.
While poets often make stanza divisions by counting lines into groups of equal length, they also create line groups of irregular length, which function much like paragraphs in prose. In fact, these groupings are often called verse paragraphs. A few short verse paragraphs could be followed by a longer one, then another short one, and so forth. This more organic and flexible approach is especially suited to free verse because it allows the shape of the poem to evolve naturally as the poem is formed.
As you read, notice how stanzas are used in the poems you most enjoy. Then, in your own poems, try out different patterns. Experiment with numeric groupings and with verse paragraphs. For example, you could divide one poem three different ways and ask members of your discussion group to help you choose among them.