In simple words, masternode ensure that blockchain of specific coin will exist even if there are not miners. You need at least lower required amount of coins and you need a server. Server have 24/7 access to internet and on that server is your masternode. As a reward for having masternode you will get some percentage. Reward depends on coin. Some coins give you voting power as well. That mean, all masternode owners vote will some specific change be implemented on that coin or blockchain. Depending on market value having masternode can be beneficial.