Any market activity, along with its production chain, from extraction and production, to distribution and retail have incidental consequences in the social and natural environment it contacts.
These consequences, when it affects parties which did not choose to incur in these consequences (external parties), are called externalitites.
There can be positive and negative externalities, but naturally the one we hear most are the negative ones, as they should be "internalized" by the responsible party.
For example, a beefarm might bring a positive externality to fruit tree owners around it, through polinization. But the same way, the increase in bee attack in the region would be a negative externality.