It's been covered elsewhere... but the basics are as follows:
Flash(ing): Installing and replacing the standard operating system that appears on the device
Unlocking: Preparing the unit to be flashed. All units are initially locked by the carrier. You must unlock the device before you can properly flash a unit.
Kitchen: A ROM base (usually from Microsoft) along with several patches and other programs that can be added like ingredients to a stew. Once everything has been added, a new ROM is produced. Essentially, it's the same OS, but with different applications and tweaks. No two are exactly the same. It's recommended that you are familiar with the knowledge of the OS and what works best before attempting to work with a kitchen.
As a word of caution... you can damage your expensive equipment by flashing the wrong ROM or an incomplete ROM. The act of flashing a device also voids the warranty of the device (according to the carriers). If you must return a device to the store, you should reflash the original ROM and lock the device first.
Everyone is a 'newbie' once. Read and observe. You'll learn from the masters. Most of us have patience with newbies, but don't push your luck.
Search is your friend. You can find most answers by searching. However, be careful and read fully before you flash your phone. It's helpful to start reading the stickied threads and then pawing through the ROM threads. Read before you post a question. It may have already been answered a dozen times already.