Interpretation of dummy variable : Random Effects Model
As a first pass, I would interpret as with any other dummy variable's coefficient. Assuming a linear model, when two countries share a border, their export similarity index rises, on average, by 0.15 when all other variables are held fixed.

If you're writing a report on this model, you'll obviously need to discuss at length what this actually means.  The result seems intuitive: countries that share a border likely share other similarities that would lead to their portfolio of exports having more in common than if the two countries were separated by some larger geological or political distance.