Across all forms of art, there exists a mechanic that leads the observer through the work. In art and photography, there's composition. In music, there's beat. In poetry, there's meter.
Meter is the basis of organization for most traditional English poetry, though, understanding its concept also allows the reader to understand organization of free-verse poetry. To be abstract, it is the heartbeat of a poem. It creates a logic that determines line breaks and word choice, allowing the author to place emphasis on certain words to build a cohesive theme or image.
There are three types of meter to consider: accented-syllabic, accented, syllabic. Each are organized with different units. Accented-syllabic is organized by feet. Accented is organized by hemistichs. Syllabic is organized by number of syllables. The first two are more prevalent in English poetry, but the third is more common in poetry from other languages, e.g., haikus. Depending on the author or the reader's preference, one type will be preferable to the others. However, when it comes to free-verse, it's up to the author to define meter with their own conventions.
For readers, identifying the type of meter the author uses allows them to deconstruct the poem and focus on the nature of the words the author chose, which words they place emphasis on, or how certain words connect to others. Beyond that, the reader can identify other, smaller mechanics that the author uses-- like trochaic substitution, caesura, feminine ends, etc.-- in order to further understand the content and author intent.
The following reference will detail various mechanics and their uses in order to give both readers and writers of poetry an idea of how the mechanics work and why they're used.