A manufacturer part number (MPN) is a series of numbers and/or letters given to a part by its manufacturer. For example, each part of a car (the brake discs, the timing belt, etc.) will have a different and unique MPN. This allows buyers to identify which company made a particular part.
Sellers on eBay will usually include the MPN in their listing so that buyers can easily find a specific part. See our Seller Center page on MPNs and other product identifiers for additional info.