“The Overwatch lets the CS:GO community regulate itself by allowing qualified and experienced members of the community (‘investigators‘) to review reports of disruptive behavior, determine whether those reports are valid, and apply temporary bans if appropriate.”
"Prospective Overwatch Investigators are presented with an active Overwatch button in the main menu, which indicates that there is a pending case for them to evaluate. The investigators can then choose to participate by watching a replay (eight rounds’ worth or roughly 10 minutes) and selecting a verdict.
If the investigators collectively agree that an offense has occurred, a ban will be issued. The ban duration will depend on the severity of the offense and the suspect’s history of convictions, if any."
Literally copy and pasting from the CS:GO FAQ about it, will post a link bellow.