For those who know what planning poker is, the landing page is very clear.
More generally: Tech is absolutely rotten with jargon, most of which is assembled from pre-existing words that have different meanings in their original context. I don't think it does us much use to complain about this fact whenever we get tripped up by it. Nobody wants to have to always define the context of their jargon before using it in order to avoid confusion any more than they want to use XML Schema and qualify the names of their tags at all times. It would just be a horrible chore. It will be quicker and easier for everyone if we just learn these bits of jargon ad-hoc, and accept the occasional moments of mild confusion as part of the job.