The egrid was a dumping ground for a lot of different ideas, so people might mean different things by it.
The egrid partly encompassed variable physics areas, so a bot could wander into a new area of the sim and be under totally new physical laws (somewhat taken care of using internet sharing across instances on a single computer).
It was partly a way for bots to have meaningful interactions with the static environment. Ant bots could create tunnels through dirt, for instance. In the modern program, shapes would be the closest analogue. If you were to expand shapes, allowing bots to interact with them, change their shape, shoot out chunks of them to make nests, etc., you'd have this.
Last, it was a way to introduce biofeedback between the environment and bots. Waste would build up in areas of high population, causing bots to spend more nrg pumping it out. Areas after a large dogfight would have a higher-than-normal free nrg concentration that bots could feed from. Bots could lay down custom chemical "pheremones" to communicate across long distances or lay a navigatory trail down.
A cursory examination of different methods of achieving the last were outlined by me
here. But basically, it represents a huge feature area because of it's potential impact on sim speed.
