A Navigation Mesh (also known as a Nav Mesh) is basically a definition of areas that an AI-controlled unit considers passable (that is, areas that the "AI-controlled" unit is allowed to move into or across). A Nav Mesh does not include geometry that would block the player if the player tried to move through it.
Laying down a Navigation Mesh
Getting ready
Constructing a Nav Mesh based on your scene's geometry is fairly easy in UE4. Start with a project that has some obstacles around it, or one that uses terrain. The ThirdPersonExampleMap included with the C++ Third Person template works well for this purpose.
