Harm reduction principles are grounded in a reality that accepts that we humans will continue to participate in activities that invite the potential for harm or danger into our lives, and the least harmful and most proactive approach to mitigate the potential damage is by reducing the risk of harm possible to any person. We cannot predict or control for all variables. But we can control what we can control -- our own decisions and, sometimes, our environment.
Harm reduction practices can show up in the form of seat belts in cars, ear plugs at concerts, a spotter at the gym, sanitation practices during Covid-19, or substance/chemical testing for drug use, not combining benzodiazepines with painkillers or alcohol, or using filters with tobacco smoke, or walking against traffic when possible as a pedestrian, or testing alcoholic drinks for the presence of unwanted substances, or abstaining entirely.
What 'harm reduction' looks like to you will be highly unique to what you do in life, but the end goal is always to reduce the risk you expose yourself to (again, like seatbelts in a car).