Planning-poker tag is used to ask questions about this interesting planning technique. Planning poker is the method of planning used in scrum to reach a consensus on the estimation of a task.
Planning poker is a poker like game played to estimate the tasks at hand in a scrum planning meeting. The wikipedia excerpts -
Planning poker, also called Scrum poker, is a consensus-based technique for estimating, mostly used to estimate effort or relative size of user stories in software development.
The reason to use Planning poker is to avoid the influence of the other participants. If a number is spoken, it can sound like a suggestion and influence the other participants' sizing. Planning poker should force people to think independently and propose their numbers simultaneously. This is accomplished by requiring that all participants show their card at the same time.