Aptitude is your natural ability to learn or excel in a certain area. It is often the case that a person has a group of aptitudes that fit together and helps them succeed at specific tasks. For example, you could have an aptitude for math and logic. Those two aptitudes likely help you excel in probability and statistics.
There are a variety of tests that can determine your aptitudes. Some will test for whether you have a combination of aptitudes that can indicate potential in a certain field, and some test for individual aptitudes such as mechanical or spatial.