Something that's draining tires you out, either physically or emotionally. Some people thrive on spending long days with young children, while others find it to be completely draining.
Running a marathon or a triathlon is physically draining — when you're done, you're exhausted. Having an argument with a family member can be just as draining in a different way, leaving you depleted and fatigued. The adjective draining uses the figurative "exhaust" meaning of the verb drain, which dates from the mid-1600s.