The IQ test is meant to measure someone’s IQ. To a certain extent, it might show the capability of the person but according to some reports, the answer to this is no. The IQ test is not a guarantee that someone is actually smart. Do take note that those who have high IQ may not always use their IQ for good things.
The average score that people get for their IQ is usually 100. Any number that goes beyond that is considered to be exceptional. Those who get a score lower than 70 are usually considered to have some mental problems or may be suffering from delayed learning.
It depends on which test and where you are taking it as to its accuracy. There are so-called IQ tests online. Take one for fun, by all means, but don't agonise over the results too much. If you are paying for an online IQ test you should examine what kind of organization is offering it. To be sure of an accurate IQ test result, ideally have an individual assessment from a licensed psychologist who will administer it and score it in a standardised manner, and afterwards give you the results and explain them thoroughly, showing your strengths and weaknesses.
A group administered test given in a properly regularised situation, such as a university, should use a widely recognised test with good reliabiity and validity. If you have taken this test in a normal manner, i.e. not ill, you should able to rely on the results.