In hypothesis testing, we talk about two important ideas:

1. Null Hypothesis ($H_0$): This idea says that there is no effect or difference. For example, if we are testing a new medicine, the null hypothesis might say that the medicine does not help people recover faster.

2. Alternative Hypothesis ($H_a$): This idea says that there is a real effect or difference. Using the same medicine example, the alternative hypothesis would claim that the medicine does help people recover faster.

The main difference between these two ideas is what they assume. The null hypothesis ($H_0$) believes there is no connection between things. On the other hand, the alternative hypothesis ($H_a$) thinks that there might be a connection.