PM2.5 is particulate matter with a size of 2.5 microns or less, for example dust, ash and smoke particles. These can be released from a range of household activities such as burning candles or cooking.
When you see PM2.5 on your machine's monitor or in your MyDyson app, you're experiencing a rise in PM2.5, and your machine is working hard to remove the pollutant from the air, and project cleaned air into your home.