Imagine that you have estimated something about your data that you don't know for sure. Assuming that what you have imagined is true, what are the chances of getting the estimations that you found or even more extreme values? This is hypothesis testing. Statistical hypothesis testing (or simply, hypothesis testing, HT) is the name given to a set of well-known, practical methods used to make inferences with statistics. As long you have data and you're willing to make some inferences about it, the odds are that HT is the way to go. It can work out a great variety of real-world problems.
Although it's usually better to work with experimental data, it's also possible to statistically test hypotheses using observational data as well. Exhibit A: economists all over the world are doing it. A medical treatment's effectiveness, production...