The interaction cost is the sum of efforts — mental and physical — that the users must deploy in interacting with a site in order to reach their goals. Zero interaction cost and is the holy grail of usability as a field.
Most of the time, users have to look around, read, possibly scroll, find a promising link, click on it, wait for the page to load, and then repeat the process all over. In other situations, users may need to remember information on one page and apply it on a different one. All these actions require cognitive effort and make up the interaction cost. These actions deplete the mental energy / willpower (see Willpower).