A scientific question is a question that may lead to a hypothesis and help us in answering (or figuring out) the reason for some observation. Asking a scientific question is a part of the scientific method (which includes making an observation, asking the question, forming a hypothesis, testing it and accepting/rejecting/modifying the hypothesis). A good scientific question has certain characteristics. It should have some answers (real answers), should be testable (i.e. can be tested by someone through an experiment or measurements), leads to a hypothesis that is falsifiable (means it should generate a hypothesis that can be shown to fail), etc. For example, an observation that a passing car makes a lot of noise can prompt someone to ask "if there is some correlation between the speed of the car and the noise it generates", which can result in a hypothesis that at higher speeds more noise is generated. This can be tested by driving a car and measuring the noise level and these results can help us accept or reject or modify the hypothesis.
Asking a scientific question is part of the scientific method. The first step is to make an observation. The next step is to ask a scientific question. After the scientific question, you form a hypothesis and conduct an experiment.
Observation: The plants in the moonlight are growing faster
Scientific Question: Does moonlight make plants grow faster?