Merchant account processing is a service provided by many banks and financial institutions and basically means that you can accept payments made using credit and usually debit cards. So in addition to the usual methods of payment you will be able to offer your customers the option of paying by credit card.
It’s not the easiest of things to set up which is why you will almost certainly need to employ the services of a professional or at least someone who has experience setting these accounts up. If you have a website, you may find that your provider can help you arrange a merchant account.