A 3M mask is a mask made by the US conglomerate 3M. It’s not any specific type of mask, as 3M makes many different models. 3M masks can be both what the 3M calls “masks” (face masks that would qualify as medical devices, like surgical masks [insert article link]) or “respirators” (face masks that would qualify as PPE, like FFP2 masks [insert article link]).
At the beginning of the Covid-19 pandemic, when the global mask supply was limited, many people used any possible mask they could find, incl. PPE you could buy in DIY stores. 3M has traditionally supplied a lot of these in many countries around the world. The 3M masks you might find at your nearest DIY store are probably comparable to FFP2 masks [insert article link] if you’re in Europe or N95 if in the US. KN95 [insert article link] imported from China can probably also be found that all look similar to the mask models 3M produces.