Stress tests are a pretty simple concept. A stress test can be both synthetic or real-world, but most of the time stress tests are synthetic for the sake of controlling as many variables as possible. Stress tests are designed to test component stability, this is usually done when applying overclocks/underclocks or a component fault is suspected.
The idea behind stress testing is to create a maximum load on a component and sustain that load until an issue presents itself, or the component passes the stress test.