1. To move or run smoothly with unbroken continuity, as in the manner characteristic of a fluid.
2. To move with a continual shifting of the components.
3. To proceed steadily and easily.
4. To exhibit a smooth or graceful continuity and smoothness.
It's important for a poem to have a sense of flow to it, one that, in a successful piece, helps to move the reader along and through the poem. If the poem lacks flow, it destroys the coherence and connections within the poem. It will feel disjointed and the reader will inevitably, feel disconnected from the original intent of the piece.
Flow is achieved through the proper use of line breaks and syllable count, word choices and punctuation. Try reading your piece out loud. Are your line breaks in logical places, where you normally pause to take a breath? Do the words you've chosen sound good together or are they disharmonious? Does the punctuation you've used indicate a pause or full-stop in a place that causes an unexpected halt?
Of course, there will be times when you choose a subject matter that may be enhanced by the use of discordant word choices or line breaks, where you want the line breaks to reflect a conflict within your poem. This is also a common usage for 'flow' -- by purposefully doing without it.
In either case, make sure that flow is something you're consciously aware of as you're writing. Make the choices based on your subject matter, and be informed about the effect of your choices.