A Maker loan allows you to use your existing Ether (ETH) to get a new asset, Dai. It lets you do this without selling your Ether.
To take out a Maker loan, your collateral (the ETH you're temporarily locking up) has to be higher than the Dai you're getting.
So why would you want to lock up $1,000 of Ether to only borrow $600, for example? Why wouldn't you just sell your Ether and use the proceeds instead?
You might not want to sell your ETH because you think its value will rise in future. So instead you lock it up with Maker and in return get an asset, Dai, that is designed to stay stable in value. You can then use this Dai a bit like traditional money.
Think of it a bit like a mortgage on a house.
A bank will lend you money based on the value of the house. You can use this money to refurbish it, buy a car etc. And you do this without having to sell part of your house. You then pay the mortgage back later.