ABC is a reference to fundamentals, i.e. "your ABC's." It just means a very basic, tight, textbook style of poker with no fancy play or adjustments. Beginners should learn and adhere to an ABC style until they have the experience to advance their game beyond that stage. An ABC style is generally profitable at the micro to small stakes over a long term, but as you progress in stakes and go up against better players, it can be easily exploited.