A node is a special software running on a computer. Any blockchain is a set of its nodes.
The nodes store data and determine, if the transaction should be executed.
In short, a computer with running blockchain software is also called a “node”.
The node of Ethereum (the most popular blockchain now) is written in the Go language.
Perhaps you use tokens and cryptocurrencies and never use the “node”. But your crypto wallet always interconnects with the node operated by wallet provider.
When you place an order to develop Ethereum application, the size of the node must be taken into account. The Ethereum-blockchain node size has exceeded 1TB, and when deploying the node you should download the data and then to process it. It will take a lot of time even on computers with large RAM and SSD disks.