Halos are clusters of galaxies and in simulations they are clusters of particles. Learn more about what they are and why they are important.
We aren't talking about a simulation like goat simulator. We are talking about a piece of software that allows cosmologists to watch the universe evolve over a period of time.
In every simulation there is an algorithm used to group certain particles and halos together. ROCKSTAR is an algorithm that does both.
This is a simple halo finder that groups halos together using newtonian gravity. Learn more about halo finders and what they do.