Here are a few pages you might want to reference during and after the tutorial, or in your further reading. Simply put, a navmesh is a collection of polygons that tells an actor where it can walk. When an area has been built and cluttered, we need to determine where NPCs and creatures can navigate.
1.3 Step 3: Navmesh the Rest of the Level.1.1 Step 1: Creating a Navmesh Triangle.