A nice feature of STORY points is that each team defines them as they fit. One team decide them as an ideal day of work (i.e. a day without any interruptions whatsoever-no MEETINGS, no email, no phone calls and so on.). Another team many define a story point as ideal week of work. Yet another team may define as a measure of complexity of the story. The most useful is estimating by complexity. For This we use one reference story and on the basis of that we estimate other user stories. Suppose if the user story is 5 points then and If any user story looks big or complex then reference story we estimate as 8 or more story points and if it looks less complex, we may estimate it as 3 or below story points.

A Story Point is a relative UNIT of measure, decided upon and used by individual Scrum teams, to PROVIDE broad brush relative estimates of effort for completing requirements stated in User Stories 

Story points further used to calculate the velocity of the sprint which is very helpful to predict the release dates.