info_node
info_node is a point entity available in all 
GoldSrc games.
info_node' are essential for three things:
- 1. It helps monsters navigate within complex geometry.
- 2. It allow some monsters to use their advanced behaviors like taking cover for example.
- 3. Nodes have a maximum connection distance of 720 units apart
The server load or (re)build and save a 'node graph' based on various information such as the positions of info_node in a map, the links between them, weigh those links to determine the fastest route from point A to B via the nodes and many more.
For flying and aquatic monsters, use info_node_air instead.
Notes
To check your info_node placement, compile the map and run it. From the console, you can use the impulse 196 and 197 commands to see a glowing path of node tracks for large monsters and human monsters respectively. This will show up any weak points in your placement and areas that may need more nodes.
This entity has a "hint" attribute but it's not used by Half-Life monsters. It was supposed to give hints to the bullsquid towards human blood. Same for the houndeye with blinking lights, machinery, alien and human blood.
Likewise, an activity attribute exists but it's not used by Half-Life monsters. It was supposed to tell the monster to perform a specific activity when it was ordered to.