Most of the time, you will get a better grasp on their definition and what they actually are if you are able to calculate these values using a 2×2 table.
Unfortunately, in real life there are hardly any 100% accurate tests, which is why you will have a lot of false positive and false negative results. On top of that, sensitivity and specificity tell us how often a test is positive in patients who we already know have the disease or not. In practice, we however do not know whether our patients have a certain condition or not. What we rather do in practice is to interpret the results of a positive or negative test.
You usually want know what the probability is that the patient actually has the disease with a positive outcome and how high the probability is that a patient does not have the disease with a negative outcome.