GL_ARB_multi_bind is mandated by the OpeenGL 4.4 specification and is a way to provide efficient multiple object binding. Per the OpenGL.org registry specification it provides a set of commands to allow applications to binds or unbind a set of objects within a single call.
In the patch-set announcement Fredrik Höglund wrote, "When binding 32 textures glBindTextures() seems to be about three times faster than calling glActiveTexture() + glBindTexture() in a loop. When binding 4 textures it's about twice as fast. I hope to land this series this week if there are no major issues."
The GL_ARB_multi_bind support is just the latest of many GL4 extensions being worked on by Mesa, mostly for the Intel Linux graphics driver.