| Soldiers fight better when they believe in what they're fighting for. High morale leads to harder performance, harder endurance and harder sacrifice. The job of political officers is to motivate soldiers, boost morale and make men want to fight and die for their ideal/country/Fuhrer.
On the Eastern Front in World War II, conditions for Red Army and Wehrmacht soldiers were hard. Very hard. This great conflict of two ideals was possibly the most ambitious invasion of all time, and millions perished. The Red Army was continually encircled, massacred and the survivors sent to Nazi forced labour camps. If they were liberated, they were treated as traitors by their countryman, and would most likely end up in a Gulag.
Stalin's secret police, the NKVD, acted (among other roles) as the political officers of the Red Army. As typical of the Stalinist regime, their approach to their job was heavy-handed. Special NKVD units were set up to shoot any Red Army soldier who dared retreat. Their job didn't just involve shooting their compatriots, though: political officers ran the Army's newspapers, which extolled the virtues of the Army and spread propaganda to improve morale.
Each batallion had a political officer assigned. The job of these officers was slightly more cheery than the NKVD death squads - these men were dedicated Communists who served to indoctrinate the soldiers with the principles of the Party. If a soldier had low morale or was voicing defeatist views, a political officer would have a quiet chat with him. If he persisted, he would most probably get a quiet death. Voicing defeatest views was strictly against the political law of the Soviet Army, and scaremongers were seen as a danger to the Army's morale. | 
