Amazon 3P or Third Party is selling products on Amazon as a 3rd party merchant.
Amazon 1P or First Party is selling products to Amazon. These will ultimately be sold by Amazon, not you.
Brian Roizen is the Cofounder and Chief Architect of Feedonomics, a full-service feed optimization platform that optimizes product data for hundreds of channels. He has been featured on numerous podcasts and eCommerce webinars, and regularly contributes to Search Engine Land and other industry-leading blogs. Brian graduated summa cum laude from UCLA with both a Bachelor’s and Master’s degree in Mechanical Engineering.