A kiriban is a set number of pageviews that somebody sets and if a person is able to take a screenshot of it, the person who set that 'kiriban' will usually draw for the person who caught the pageview.
For example; say my kiriban was 21,996 pageviews. Somebody caught it, took a screenshot of the number, sent it to me, i'll draw something for them in return.
Ahhh sorry if it sounds confusing! >U</