A navigation node for ground moving NPCs. Navigation nodes are baked into the nodegraph so that NPCs can move to them. Ground nodes fall to the ground when they spawn, so you can place them up to 128 units above any solid world geomatri, which excludes func_bush, and they will still work.
Create an interesting map, place some enemies in it, and run it. Don't kill them straight away but turn on mat_wireframe and watch their actions as you move around. You'll see them, more or less, run straight at you and shoot. This is because they have no bearings in the world other than your position.
Leave the game and lay down info_node entities across the map, preferably at each point where the AI would have to choose what to do, or anywhere the AI might want to go- basically, any important place for the AI. Then recompile and do the same as last time. You should now see them group and attack together if they need to, seek cover, throw grenades at far more suitable times, try to get around you and more - all because you added a few nodes.
When placing nodes on displacements, place them slightly above the terrain to ensure connections are properly made. Use the Transform tool (Ctrl+M) to move many nodes quickly.
Example map (VMF) (http://www.btinternet.com/~varsity_uk/VDC/Source_AI/info_node_demo.zip)
This alone has fully set up your AI for a map.
<output name> <targetname>:<inputname>:<parameter>:<delay>:<max times to fire (-1 == infinite)>