Think of it as making a meal in the kitchen. You're ROM (Hard Drive) would be the cabinet in this scenario. Sure it holds a lot, but it takes a while to get to. Ram is like your counter, you may not be able to store as much crap there, but it's a lot faster to get to.
Let's say you wanna make some food (or do an action on your computer), first you take out of the cabinets (the HD is read) then it's loaded into the RAM (Random Access Memory) which is a lot faster to get to, then it will be processed.
If you had no ROM on your computer technically it wouldn't boot up, it wouldn't even post. Your harddrive is rom, but technically the CMOS has ROM on it as well, which stores the BIOS data.