Form concerns shape, that is, the look and layout of writing. Sound is also an aspect of form, so when you consider the form of a text, you are thinking about aspects of both its shape and its sound. Form is important because it helps to shape meaning in subtle ways.
It is important to recognise that form often conditions the way we respond to a text. When you see a text in the form of columns with headlines, its form 'tells us' it is a newspaper article. We are conditioned to respond to this form in important ways. We tend to trust it, for a start. The form of some leaflets is important - some you will put straight in the waste bin, others - such as a health or charity leaflet - you will probably read with care. It is the form that helps you decide.
The most conscious and obvious use of form is found in poetry. Only poetry allows the chopping up of a sentence into lines, for example. Also poetry relies heavily on the different forms of sound words make (such as rhythm, rhyme, alliteration, assonance and onomatopoeia). All of this is used by the poet to add to the feelings and meanings being created.