The nature of an Eclipse project is a concept defined by an Eclipse Platform which allows a plug-in to tag a project as a specific kind of project. Intel(R) Software Guard Extensions uses an SGX nature to add SGX-specific behavior to projects. Project natures are defined by plug-ins, and are typically added or removed per-project when the user performs some action defined by the plug-in.
To use Intel(R) Software Guard Extensions Eclipse Plug-in in your project, you need to add SGX nature to it. You may either add SGX nature to a pre-existing C/C++ project or create a project with SGX nature from start. See Adding SGX Nature to a non-SGX project and Creating a New C/C++ Project with SGX Nature for how to complete these tasks.