It's where USA and Britain came in to assist Russia. They gave USSR military, strategic and food aid. 1/3 of all USSR vehicles came from abroad. By 1944, Land Lease contributed 10% to USSR GDP. They also helped out with allied bombings of Germany and battles of the '3rd front' made German forces overstretched.
It was aid given to the Russians starting in 1941 as Roosevelt felt he could trust them.
It supplied 12%of aeroplanes, 10% of tanks,2% of artillery to the Russians and only 4% of industrial goods came from it. It was more useful for giving food and transport to troops by 1945 the Red Army got 200g of American food a day. 1900 trains and 2/3 of jeeps were American. It showed Russian inadequacy in producing certain goods so much so that praising foreign technology became a crime in 1942.