Why are the Propo's so vital to the rebellion?
"Propo" is probably short for "propaganda." The rebels in Mockingjay use propaganda such as video clips and special publications to spread the message of the rebellion. These propos encourage other people in the districts to rise up against the government and demand change. The propos are visible proof of the rebellion's existence, and make it easier for the people of Panem to relate to the rebels. The propos are the encouragement Panem needs in order to make change happen. Otherwise, the rebels would have no support, resources, or power. Unless the rebels are able to convince Panem to join them, they pose no threat to the tyrannical government.
As the Mockingjay, Katniss herself is a piece of propo because she gives the rebellion a face that people recognize and are eager to follow.