What is an orbital boundary surface?
It means that the boundary surface is a closed surface or set of closed surfaces, such that there is a 90% chance that an electron is interior of the closed surface and a 10% chance that the electron is exterior of the closed surface.

A better way of specifying how an orbital is represented is "equal probability boundary surface" which further specifies that each point on the surface must have any equal electron probablity density, and the surface must be a closed surface or set of closed surfaces with a 90% (or other percent) chance of containing the electron at a given time.  

Without the "equal probability" statement there are infinite different boundary surfaces for each orbital (infinite different ways to enclose 90% of the probability density).