Commissar's were political officers that represented the Communist Party on the battlefront. They worked in a system called dual command in which all military generals shared command with a CPSU appointed commissar. They were around from 1918 - 1924, 1937 - 1940, 1941 - 1942 before being abolished. However, the Red Army retained the function of political officer up until its dissolution.

The role of a commissar was to inspire the soldiers to fight for Communism whilst also keeping an eye on them and the general for the CPSU.

Source: Court of the Red Tsar by Simon Sebag-Montifore.