Adafruit library

I am trying to use Adafruit_GFX library but I don't understand how I can find what KEYWORDS I can use, what functions includes, what parameters I must include in function, what the description KEYWORD1 or KEYWORD2 are describing. Is there any way to learn how to manage my ignorance about those ? Trial end error did helped me for some cases, but is too time consuming and disappointing way!
Any article maybe ?
Thank you for any help provided.

The library has one or more .h file that describe the interface

Adding to Rob's post, the class is a KEYWORD1. A method or function in that class is a KEYWORD2. Here is an example using the GFX library with only a couple functions added.

Adafruit_GFX KEYWORD1

drawPixel KEYWORD2
drawLine KEYWORD2