Private Military Company = private militia. They've been used by other countries, too, eg the US used Blackwater in Iraq. https://en.wikipedia.org/wiki/Blackwater_(company)
They could be doing anything from providing security guards for buildings, to being private armies not covered by government oversight or subject to military rules of conduct (so they're notorious for atrocities).
They offer deniability to any nation state that employs them, because the leaders can stand up in Parliament or wherever and say "Not our nation's army guv. Just some mercenaries we know nothing about."
Wagner have been active in Africa for some time, sometimes even working hand in glove with national armies there, and have been involved in many atrocities in Syria, Mali and elsewhere.
https://en.wikipedia.org/wiki/Wagner_Group
Wagner Private Military Company has been a major part of the Russian forces in Ukraine. Prigozhin, owner of Wagner, has also been the owner of the "Internet Research Agency" – a company of Russian troll farms which (very successfully) try to steer public opinion across the world in directions Russia finds helpful. The troll farms are very active during US elections, for example, and during UK referenda (Scottish independence, Brexit). But they also do a lot of purely disruptive work, turning up the temperature and trying to get people at each other's throats; any disruption may provide an opening Russia can then exploit.
If Prigozhin loses this face-off with Putin, then not only do we have to consider what the removal of Wagner's boots on the ground will mean, but whether they will drop out of the information space. It has already been possible to see a shift in troll behaviour because, a few years back during the Salisbury poisonings, it was noticeable that trolls could never criticise Putin. In the last year or so, they've started saying things like, "Obviously Putin's an idiot" – while still churning out Russia Good, Ukraine Evil material.