Nodes and masternodes are components of the network for a particular cryptocurrency (coin or token). They are computers/programs that execute a variety of duties, and some of them contain complete ledger copies. If you operate a node or masternode, you are typically compensated in the form of the masternode's coin or token. This can be really lucrative. However, setting up and maintaining a node or masternode is typically quite technical and requires specialised hardware and a consistent power source (or a VPS). Yieldnodes performs all of this technical work and splits the profits with you.