A hypothesis is a specific statement about how something behaves that can be checked against evidence. In quantitative trading and financial research it is the starting point of a study: a claim about a market pattern, such as "shares that beat earnings forecasts keep outperforming for several weeks", written down before the data is examined. A useful hypothesis is precise enough that data could show it to be wrong.
In statistics the claim is split into two parts. The null hypothesis says there is no effect, and the alternative hypothesis says there is one. Hypothesis testing then asks whether the observed result would be surprising if the null were true. Stating the idea first matters in markets, because searching price history for patterns and explaining them afterwards leads to overfitting. A sound research process moves from an economic reason, to a hypothesis, to a backtest designed to challenge it.