"installd" is a completely undocumented (outside of Apple) daemon that is somehow involved in installing your software (it runs while installing App store programs) and checking to see what updates you might need. It has no user visible existence. No one would ever know about it unless they ran "ps" or something similar. They will find no hint at what it does, or if killing it is safe.
installd is part of framework that handles installed packages. It's fired up as part of the normal software update process (although it really shouldn't be taking a core for itself). Killing it would probably abort the software update process.