Benjamin Franzke, an independent open-source developer, has published patches that provides Wayland EGL support for Mesa. This provides a EGL Wayland platform library so that it can be used by this new display server as an alternative to X...
EGL is an API to talk to a windowing system to get a rendering context.
It can sit directly above the hardware drivers. This is what Wayland has been using. It lets Wayland get a rendering context from Gallium so it can actually composite windows to your display framebuffer.
The actual applications need to get a context too, though. On X, they use GLX to ask the X server to return a context associated with an X window. On Wayland, GLX does not exist or even make sense. So some other API needs to be used by client applications. Wayland could invent some new proprietary context API, like GLX or WGL or AGL. Or it could just use this neat existing API called EGL that it's already using itself.
So EGL can be used by client applications to ask Wayland for a rendering context. That means that the system EGL library needs to know how to ask Wayland for a context. That's what this patch implements.
EGL can already be used over X11 or Windows or so on. EGL requires some small glue to connect it to the native system (be that a complete windowing system or a low-level driver/framebuffer abstraction like KMS). This is just the glue for Wayland.
Or, as was said in the e-mail linked to in the article, a header file was added (to wayland) which may be used by other programs that wish to use EGL/Wayland. The header defines what applications have to implement. It interfaces with a corresponding library (the patch sent to the mesa-dev list) in mesa. (paraphrasing/making things up as I go, source: mesa-dev, mesa-patch)