Hi there,
MasterNode are really a fashionable trend at the moment.
Basically, a MasterNode is a node that keeps the blockchain of a particular coin. So it means that it participates in the block mining, and thus contributes to the coin blockchain.
Each time a Masternode mines a block, its is rewarding with a certain amount of the blockchain coin.

Some of the MasterNodes on the market can be good investments, as they have a real reason to exist.
Others are more shady, and you should beware of them, as their only reason to exist is that people buy the coins to create MasterNodes with the hope to have a ROI.