Seasoning a pixel is a term used to refer to a pixel that has gotten a certain amount of data to it, and Facebook can now to some extent give you more accurate results based on those particular kinds of people who have visited your website.
For example, if I am selling a Real Madrid jersey, I want to get people who buy those Jerseys, not just people who like or comment but never buy.
And the best way to get these people is to get a person to move from Facebook to my website. Each of these people is recorded by facebook, based on a code(The Pixel, more like Google analytics code) and reported to me. The more the number, the better.
From that, I then start optimizing my ads based on specific actions taken on my website. The more the number of people who visit my website, the more accurate facebook will get in giving me the people most likely to match those taking specific actions. So thats seasoning a pixel, getting more people to take more specific actions on your website to help you optimize your ads as much as possible for the best results.