Planning poker is a technique for estimating the effort required to implement user stories. It's commonly used in agile software development, especially in Scrum, and involves the whole team.
During a planning poker session, a user story is picked from the backlog and presented to the team. Each team member estimates the story in secret by assigning story points to it—usually, Fibonacci numbers are used. When everyone is ready, the estimates are revealed simultaneously. The team is then given the time to discuss differing estimates and the rationale behind them. This procedure is repeated until everyone arrives at the same value.
Planning poker tries to combat lengthy avoidable discussions and anchoring—which is when the first estimate uttered by one person influences estimations of the rest of the team. Time-boxed, rigid structure is meant to address the former, while revealing the estimates only when everyone's ready tackles the latter.
However, this technique is not without its flaws. It's hard to give a concrete estimate without any point of reference, especially if the user story is outside of one's domain. This leads to discussions that focus more on what particular story points represent, rather than the user story itself. Moreover, estimating in precise values compels the estimator to focus too much on implementation details.
There are relative estimation methods like the team estimation game that address these issues and, in our opinion, achieve the same result more efficiently. We thus advise to consider using them instead of the planning poker.