The meaning of form_build_id Can you tell the meaning of form_build_id

I am trying to solve problem "add to cart form breaks after using Exposed filter with Ajax"

As I investigated, only form_build_id value changes when we are trying to search product catalog for certain products with Exposed Filters.

If I take initial form_build_id value for product's add to cart form, search catalog with Exposed Filter, than substitute via Firebug new form_build_id value with initial, and the form works
The form_build_id is used as key to cache a particular build of the form. For multi-step forms, this allows the user to go back to an earlier build, make changes, and re-submit.

See also : drupal_build_form,  drupal_rebuild_form