There are many long and intricate definitions of 'harm reduction', but put simply:
Harm reduction is a way of working with people who use drugs that aims to reduce the risks those drugs cause to both the person using them and society.
Harm reduction is often seen as interventions like the distribution of sterile needles, or providing safer spaces for people to use drugs. However it also includes drug treatment; support and counselling; and the people who work to promote and encourge changes to oppressive drug laws. The easy way to think about it is to understand 'seatbelts are harm reduction for driving' - they don't stop people driving badly or having accidents, but they help keep people alive when accidents happen.