Marketable things are in a position to be sold, or are in demand in some way. A marketable employee is one that potential bosses want to hire.
If you graduate from college with a degree in engineering or a nursing license, you will probably be marketable — employers will want to hire you. You might worry that you'll be less marketable if you major in philosophy or French literature, but in certain fields you will still be quite marketable. You can also describe anything that people will want to buy as marketable: "Your spinach-flavored ice cream idea just doesn't seem very marketable to me."