Poetic form is the rules by which a poem is written when the poet's desire is to present the text following a specified meter, rhyme, rhythm, or use a specific poetic device.
Poets can follow rules specific to heroic couplets, Petrarchan Sonnets, or Italian sonnets (just to name a few). The rules of the poem's form bring out specific meaning for the text, the way a poem is meant to be read (as in Shakespearean poetry--Shakespeare wrote the way that he did so that it would mimic human language), or the imagery it was meant to depict.
For example, a shape poem uses the shape of the object it depicts and the words form to the shape of the object depicted. A poem about an apple would look like an apple from far away and the words of the poem form the picture of the apple. The meaning of the poem would relate to an apple and the form would provide the imagery.
Therefore, the form of a poem can help to bring out the meaning in a poem if it follows a traditional known form which readers are familiar with so that they know how to read the poem and what to look for.
Novel poetry readers will find it difficult to make out how the form of a poem adds to the meaning given they do not have enough knowledge about the reasoning behind the use of any specific poetic form.