Every time 840,000 blocks are mined, the reward given to the miners is cut in half. This means they receive fewer Litecoin's for their work. This process, called block reward halving, controls the number of new Litecoin's entering the market. Reducing the rate at which new coins are created, it helps prevent inflation and makes Litecoin's scarcer. The idea is that as the supply of Litecoin's decreases, their demand and value will increase.