In very brief terms - The head of Russia's largest affiliated private military organisation (Prigozhin - the Head of Wagner PMC) has declared war on the Russian Ministry of Defense. This is significant as Wagner were a major component of Russia's offensive in Ukraine.
This is a culmination of many weeks of Prigozhin calling out the Russian military for their incompetence. Important to note, he hasn't directly called out Putin. His truck seems to be with the Defence Minister (Shoigu) and other high ranking generals.
The Prosecutor General has now ordered Prigozhin to be arrested for armed insurrection.
These are the facts, the rest is speculation. That speculation being that Russia is now locking down its major political institutions with military security. There are some indications that Wagner are going to try and fight their way into Russia, but the truth is nobody even really knows where they are right now.
If you want to know what Russia was doing in cahoots with a private military organisation in the first place, that's a whole other story.