RFID stands for radio frequency identification. In a nutshell, it's a little tag that emits a radio wave key code every time it's activated by an RFID reader. You can buy these little plastic cards that look like hotel keys that are RFID tags (You can also find them in a ton of different form factors like keyfobs or capsules). And most of these are passive RFID tags, which means they don't need a battery to operate. So you have an RFID reader which emits a constant frequency, and when you bring the tag close to the reader the radio waves that are emitted by the reader power the tag just long enough for it to send its serial key via radio wave to the reader. The reader then interprets this and sends the data to (in my case) the Arduino via a serial line for processing.
So i have this reader mounted inside my driver side window by my mirror -- in my car there was this plastic part that hid wires from that went to my side mirror, so i cut a tiny part of it out and fitted the reader into it. Ran some com wires from the reader through my door (alongside the wires from the door locks, and window controls) to underneath my steering column, where I have my Arduino mounted.
So to unlock my car, i swipe my RFID keyfob in front of the reader, the Arduino checks to see if it knows the key, and if so, unlocks my car doors.
Regarding your issue with including the file, if you put it in the library folder you should be able to include it. As I understood, keywords.txt was used by the Arduino IDE for highlighting keywords only.
Also, you're not supposed to understand everything! I don't either. It's just a complete list of EVERY memory allocating mechanism in C++. 99% of which i'll probably never use, but I'm sure the STL takes advantage of them. What's important is in new.cpp:
//this is the meat of what's going on there.
void *operator new(size_t size)
void operator delete(void *ptr)
But Atmel Studios is really nice. It takes a second to figure out how to configure it, but when you do it's soo much better than the Arduino IDE.
That's the tutorial I followed to initially set everything up.