Planning poker is a technique to estimate the effort of a Product Backlog Item (PBI), for example, in story point.
The basic idea is that for each item of the backlog, the team agrees on the number of story points needed.
This method is called planning poker because each team member uses a hidden card to suggest the story point of the item. All the teams flip the card simultaneously, so in this way, you didn’t get biased.
If the team member gives a different score, they can talk about it and then make another round with the card. This loop ends when all the team agrees on the same score.

To get more detail on it, I suggest looking at this Wikipedia page.

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