Persona is latin for mask. You can think of a persona as your users of your system each wearing a different mask. For instance you might own a company that sells outdoor supplies. One persona might be a backcountry guide the next could be a general manager at a sports store and perhaps your supplies really cater to the oil patch as well. Being able to identify your personas and who they are helps you understand the calls to action you present to your personas, the technology you can adapt with your personas and even design elements that they would give them a great user experience. 

The ideal situation for persona mapping is to first have focus groups and conduct interviews with your focus group to get a true understanding of who your customer is. For many business owners focus groups can be a cost they can skip over as the know there customers and can generally tell us a good idea of who these people are, their habits and what they are looking for. Persona mapping allow us to see into the heart of your business and make sure we extract the most important services and products for your most important persona upfront in the design of your website or software.