Hi, this stuff is really for the advanced user and not a noob (newbie), but i started out as a noob myself and now i will NEVER use a stock ROM again. A ROM on your phone is the built in software that creates the user inteface (UI). The chefs here write these roms with a base OS (ex. your HD2 uses windows mobile 6.5) and allow you to squeeze the most potential from your handheld device (I almost never use mylaptop anymore).
VERY IMPORTANT that you read the thread I'm putting right here, otherwise you will turn your beautiful device into a very expensive paperweight.
Here are two links that you need to read, if you dont understand something read it over again or ask before you attempt a cook.
this one is actually a video guide to flashing a rom (cooking)