PMS is an acronym for Pantone Matching System. It is a system that gives a unique color code ID for every color under and over the rainbow. Usually a 3-4 digit number make up a PMS color code. Some have names, such as "Process Blue". All of the colors are found on a full PMS color chart.
Some organizations brand has a specific color they use. Whenever marketing or promotional material is printed, such as promotional pens,they will require the printer to use their PMS color(s). For instance, Microsoft's blue is PMS (279). In order to keep the integrity of their logo, they require the use of this exact PMS for all printed material.
When ordering promotional pens and you want the custom pens to match your logo exactly, you'll need to know the exact color. You'll especially want to use the same color if you are buying multiple promotional products and you want the print color to match.
If you have chosen to use "PMS Match" as your imprint color, with the artist. Be sure to tell them the PMS color in the comments or however you are getting the order art details clarified. If you do not know the color, you should be able to send your logo to the company who is printing for you and they will be able to do the matching for you. Charges may apply to your promotional pens order if the company does not have your specific color already mixed and in stock.