According to Apple:

autofsd

runs automount, and then waits for network configuration change events and, when such an event occurs, re-runs automount to update the mounts to reflect the current automounter maps. It can also be signalled byautomount_reread to run automount.

More simply – it monitors for changes on the network. Let’s say you come home and connect to your home network, autofsd would be responsible for saying “Oh look, I can mount my home server’s hard drive on this network!” and take care of business.