A blockchain is simply an open, secure, and shared database.
This shared database is maintained by nodes. Nodes can be small, general-purpose devices like laptops or specialized, high-performance servers.
Nodes run the software built in/on the database, store a copy of the most up-to-date information, validate new information as it is added to the database, and help other nodes sync to the latest version.
There are different types of nodes: miner nodes, light nodes, archived full nodes, and masternodes.
As traditional networks get larger, they become more vulnerable to attack. But as a network of nodes (a blockchain) gets larger, it can improve security by having more participants validate and store the information.
Summary: A node is one computer in a network, that shares the information and enforces the logic of the network. A large number of distributed nodes means that as the information is stored on more nodes, it becomes more difficult to hack the entire network.