The blue/red bar represents the power of the guards and the prisoners of your prison. This will be reflected in your riots.
The blue section shows the guard power and the red section shows the prisoners power.
Also, the bar shows the position of power of each type of prisoner through a person icon distributed along the bar. Brown is for low-security prisoners, orange is for medium security prisoners and red is for high-security prisoners.
You can increase your blue bar by buying and upgrading items and hiring workers related to security.