A surface bar is a type of staple-shaped barbell recently developed for surface piercings. These piercings have a high rejection rate, which is at least partly due to inappropriate jewellery. Curved barbells, captive bead rings and especially straight barbells put too much upward pressure on the piercing, causing the jewellery to be pushed out through the skin. Surface bars are designed to reduce that pressure as much as possible, and therefore reduce the likelihood of surface piercings rejecting.
Most surface bars look roughly like this:
They do come in slightly different shapes, though, for different piercings. This is another possibility:
The main point of the surface bar is that, where the jewellery enters or exits the body, it should be perpendicular to the skin. This reduces the pressure of jewellery against skin to a minimum.
Surface bars can theoretically be made out of any metal, but some (such as silver) are not considered suitable for piercings. Also (as I recall), a recent EU directive states that body piercings within the European Union should be carried out using jewellery with an absolutely tiny maximum nickel content (I forget the precise number); in terms of cost, this basically means titanium is the only possibility. Surface bars often need to be custom-made; everyone is built differently and doing a piercing to fit the jewellery in stock, rather than finding jewellery to fit the piercing, is asking for problems.
Another option for a less problematic surface piercing is a flexible barbell made out of tygon.
Everything I ever needed to know about body piercing I learned from BMEzine.