Wagner is the mercenary organisation, and an utterly vile one, that Putin's used regularly. Since merc organisations are illegal in Russia (!) they aren't ~supposed~ to be in Russia itself.
Wagner and the official Russian forces have been at loggerheads for a long time. Putin played one side off against the other.
The leader of Wagner, Prigozhin, has been more and more openly critical of the official Russian MoD and even taken potshots at Putin.
As Wagner was getting more and more powerful, the MoD finally persuaded Putin to make all mercenary organisations sign a contract with the MoD to be officially part of their forces.
Prigozhin refused. His erstwhile allies the Chechens did sign the contract.
All this is certain.
Now, tonight, Prigozhin claims the MoD forces fired on a Wagner camp and therefore he's hopping mad and getting violent against the MoD with talk of marching on Moscow (!)
No one really knows what's happening but something certainly is, because TASS has mentioned it and that's the official Russian news agency.