Brief Summary
The Set-Up
The allies of the United States needed help, and Congress decided it was time for the President to be given the power. Short of declaring war, America's industrial centers would be the lifeline of those fighting Fascism.
The Text
The Lend-Lease Act gives the President the authority to decide which countries need material support, what to give those countries, and how that material support will be repaid, if at all.
TL;DR
The President can decide who gets stuff from America, and what a fair repayment is.