A Discipline is a business rule intended to reduce financial risk. For example, Chase has a Production Right that restricts certain Users from ordering above what has been estimated on a CE. Although it is simpler to use Production Rights, they do not allow exceptions. A discipline can be created to allow users to request an override (ie. approval of an exception to the rule) for transactions they don't typically have rights to perform.
Disciplines are similar to Workflows, however Workflows do not allow overrides.