We experience and represent the world through our senses (modalities in NLP): Visual, Auditive, Kinaesthetic, Olfactive/Gustative (VAKO). Within each of these senses there are a number of distinctions (called sub-modalities in NLP). When people think (internal representation/dialogue) and talk (verbally) they use these systems.
When people express an experience verbally it is possible to identify the representational system they are using internally by paying attention to the modalities of the words being used. Usually people will tend to have a preferred representational system, so they will tend to privilege this modality in their expression.