Garfinkel has a wordy, rather technical-sounding description of the study of the taken-for-granted, the made-practical, the reasonable, etc. It's those unseen practices for making actions, events, etc. normal-ish, that he wants to study. And because it is so normal, and so intentionally ignored, by the 60s it still had not been a subject of rigorous sociological work.
It's a classic, but also a headache. Good luck!