Earnings season (roughly 4 weeks per quarter, starting in mid-January, April, July, October) is when companies report their quarterly results. These reports are the most reliable catalyst for large stock moves.
An earnings play involves buying or selling before or after the report. Pre-earnings plays bet on movement. Post-earnings plays trade the reaction. The risk is different in each case.
The EPS surprise is the most watched metric: did the company earn more or less per share than analysts expected? A big positive surprise often results in a gap up. A miss often results in a gap down.
However, stocks do not always react logically to earnings. A company can beat expectations and still sell off if guidance is weak or if the stock was already priced for perfection.
For beginner swing traders, the safest approach is to wait for the earnings report, see the initial reaction, and then decide whether the move has legs. Buying the day after a strong earnings gap is a common strategy.