When a master limited partnership such as Eagle Rock Energy Partners or EV Energy Partners performs badly over the course of a few years, you can understand why a ratings agency such as Standard & Poor's would rate it non-investment-grade, or junk. But what about an MLP such as Martin Midstream Partners? It's had a great run this year, and it might surprise investors to know it's rated junk, just like the other two. In this video, Fool.com contributor Aimee Duffy looks at the rationale behind these ratings and illustrates the importance they have to investors when they're first getting to know an MLP.