What does the IP66 rating mean for the power supply?

Without getting too technical, an IP rating is shorthand for an Ingress Protection rating – how well something can stand up to the elements. The first digit is protection from particulate ingress (dust). The second digit is protection from moisture ingress (water). So the first 6 denotes full protection against dust and other particulates, including a vacuum seal, tested against continuous airflow (and is the highest on the scale 0-6).  The second 6 denotes protection against powerful jets (12.5 mm nozzle) of directed water from any direction (with the scale going up to 9). In a nutshell, at IP66 the power supply is environmentally sealed and weatherproof. It can handle the outdoor life! Just don’t submerge it in water.