Planning poker is a method of estimating the sizes of user stories during sprint and release planning meetings. Team members discuss a user story for a short period of time, after which team members assign that user story a points estimate. This is often done using Fibonacci numbers (1, 2, 3, 5, 8, 13, and so on), where the lower numbers are easier items to work on. The team members with the highest and lowest estimates defend their positions, after which team members re-assign that user story a points estimate.
The purpose of planning poker is to achieve a consensus among the team as to the relative sizing for all user stories that are candidates to be moved into a sprint backlog or to be worked on during any single sprint. The user story sizing estimates help product owners select the user stories with the highest business value.
User story size estimates are intended to show how much effort is required in order to complete the work on an individual user story. In general, user stories assigned a 1, 2, or 3 are user stories that can be completed in a single two week sprint by two or fewer team members. User stories assigned a 5 or an 8 are more difficult, may require many team members, may have dependencies on other user stories, or may be too big to complete during a single sprint. Some user stories with a 5 or an 8 sizing estimate should be broken down into smaller user stories. User stories assigned a 13 (or greater) are almost always too large to be worked on during a single sprint and should be broken down by product owners into smaller user stories.
That said, there should be no literal meaning associated with any particular number in the Fibonacci sequence other than the meaning your team learns to associate with them over time. Some teams use standard playing cards, with an Ace representing a 1 and a King representing a 13.