An opening shot is the shot that helps transition the rally from "closed game" to "open game".
The terms "open game" and "closed game" are analogous to, and possibly borrowed from, chess. In chess, an open game is one where the lines are open and the pieces can move freely. Closed game is one where pawns are blocking the lines and pieces can't move freely.
In Table Tennis, open game is where you can freely swing your arm and play what are considered "rallying shots". A closed game is where the options are limited to passive strokes (typically short game).
In professional men's game, a common shot that opens the rally is a spinny loop.
These are sub-pages of the Opening Loop page (a child in the hiararchy).