Before you add a story to a sprint it should be pointed. Pointing a story means assigning a number that represents complexity to it. Metrics like velocity are driven by these points.
Story Points
There are many different conventions for what numbers or values to use for story points. We like to use a Fibonacci sequence of 1, 2, 3, 5, 8, 13. We don't really go higher than 13 in practice. In fact, we have yet to work on a 13-pointer. The stories that we've assigned 13 points we took as a signal to break down further.
Points should be treated as relative complexity, not estimates of time. For example, if the team is trying to point Story C and trying to decide between a 3 and 5, they should compare it's complexity to the already pointed Stories A and B. If Story B was a 3-pointer and Story C is a bit more complex, then it deserves a 5.
Playing Poker
Planning Poker is a method for capturing everyone's initial estimates without the influence of others on the team. As a team is getting used to this process or working with one another, the point estimates can vary wildly, but eventually they'll mostly be harmonious. However, the real value in planning poker arises when you have say three members estimate a 3 and the forth estimate an 8.
This is when you have discussion around the delta of those scores. Does the fourth know something that the others don't know? Perhaps she has insight about additional complexity. But also the inverse could be true. Perhaps the fourth wasn’t aware of pre-requisite work done last week that makes this simpler.
Weekly Pointing Meeting
This meeting is where we take stories that have been groomed and marked Ready for Dev but don't have point estimates on them yet and play Planning Poker to quickly establish story point estimates. This meeting is typically 15-30 minutes.
It is important that this meeting not really be a place for discussion about issues. The only conversation is arguing for your estimate when there are differences.
The ScrumMaster drives this meeting and resolves differences as needed. any differences. It is important that every score amongst the development team has consensus before moving to the next item.. If consensus can't be reached, it is ok to skip it for now and make progress on the rest of the list.
Conclusion
After stories are pointed they are ready to add to a sprint that you are planning. Story points are a key element in being able to predict the future, decide what you can commit to do in a sprint, and more.