A Story Point is the measurement of complexity of a story or user requirements. Story Points have no units. Story Points do not consider the time taken to complete a task or a Product Backlog. Time taken to complete the same task by different developers are different. Some developers may take less time to complete the task. However, some developers may take more time to complete the same task. Thus, time taken to complete the task cannot depict the actual complexity of the task.

As per Scrum framework, the developers estimate the effort needed to complete a task or a Product Backlog. For this estimation, the developers use empiricism. They estimate the complexity level of a task based on their past experience. However, this estimation is quite difficult to calculate. An accurate estimation helps the Product Owner order the Product Backlogs.

There are many methods to estimate the Story Points. However, all these methods use empiricism. One popular method to estimate the Story Points is described below.

1. Developers of a Scrum team select a Product Backlog.
2. Each developer assigns story points to the task from their past knowledge. The Story Points are Fibonacci numbers (1, 2, 3, 5, 8, 13, 21 … ≤ 100).
3. The number having the highest frequency is considered as the Story Point.
4. If the highest frequency cannot be determined, the process is repeated again until it is converged.

The estimation is correct when samples are more.

More methods to estimate Story Points are given in the following article:

https://www.netsolutions.com/insights/how-to-estimate-projects-in-agile/