Actual explanation of why Hitler decided to invade Russia would be very long, so to keep it short, Hitler was invested into the ideology of Lebensraum, "living space", basically having more land.
Before the war, both Russia and Germany signed a pact, the MRP, meant to keep peace between the two nations and to help divide Europe between them. Hitler knew very well they weren't going to stick to that promise, and he had his reasons:
1. Before the Nazi party was even elected, the idea of Lebensraum became their main goal. Germany had already tried to win over territories in WW1, but failed miserably, in the end even losing land, so the Germans were quite uneasy, and when a political party came along, that promised to fix everything, people liked it. Taking over and using the resources of Western Russia for the good of the German people was a big part of Lebensraum ideology. They believed themselves to be the "master race" and every other race should be ruled by them.
2. Russia was working with the allied powers. In order to win the war, Germany had to win on two fronts - the Western and the Eastern front. The main plan was similar to the one in WW1 - win the fight over at the Eastern front, move all the troops back to the Western front to invade Britain and secure the victory of WW2.
On June 22, 1941, Operation Barbarossa commenced, and Germany's army advanced into the Soviet Union. The result was the most bloodiest conflict known to man - estimated over 30 million lost their lives in the Eastern front. 80% of all German deaths occured there, and this loss served as the main reason for the defeat of Nazi Germany in WW2.
