Leasing mean someone will delegate their steem power to you and you pay them a fixed payment (higher than what you have got) after a month. You will use that Steem power to maximize your earnings during the lease period. For e.g - Assuming you buy 1000 steem Power and promise to pay back 1200 steem Power after a month. You would use that Steem Power to gain 300 steem Power over the month, so net net you would gain 100 SP.