Seems like you should have done the research first. Don't put the cart before the horse. Spend some time reading up on rooting and custom ROM's. In a nutshell, ROM Manager provides an interface for downloading/backing up/loading/etc ROM's.
It doesn't really affect your phone on its own. You use it to perform the functions mentioned above.
it's just an app to assist with functions for rooted users, like dl/flashing roms, themes, other zip files, etc. it doesn't really do anything that you can't do yourself thru recovery - it just makes doing those tasks easier.
personally, i rarely use RM. i dl and flash all my roms and themes and other stuff manually in recovery. it's more reliable that way, anyway.