The application of a science experiment is how the knowledge can be used in the real world to address everyday problems. For example, Alexander Fleming learned through experimentation that the mold penicillium rubens produced a substance that inhibited the growth of staphylococcus bacteria. The application of this knowledge was the development of an antibiotic, penicillin, to treat bacterial infections. Sometimes the application of scientific research is more of a by-product and not the original intent. The microwave oven, for example, is an application of research originally intended to produce combat radar equipment.
Science Fair projects often have a requirement to state an application based on the outcome. If this is the case it's a good idea to choose a topic for which applications are more obvious, regardless of the outcome of the experiment. You usually don't have to actually produce the application, just recognize and suggest it. For example, if you decide to test various materials for their ability to absorb oil some applications might be developing a better way to clean up oil slicks on water, to clean birds that fall victim to oil spills or to clean oil drips from a concrete driveway.