In the context of solar power, Standard Test Conditions is the set of criteria that a solar panel is tested at.
Voltage and current produced by solar panels change based on the panel temperature, intensity of sunlight, and other factors. Since these vary a great deal in real world conditions, all solar panels are tested to the same standardized test conditions.
The STCs include a solar cell temperature of 25°C (77°F), light intensity of 1000 Watts per square meter, and the atmospheric density of 1.5, or the Sun’s angle directly perpendicular to the solar panel at 500 feet above sea level.