A node is an operating system or container running Jenkins as an agent. Most work a Pipeline performs is done in the context of one or more declared
node steps. Confining the work inside of a node step does two things:
Schedules the steps contained within the block to run by adding an item to the Jenkins queue. As soon as an executor is free on a node, the steps will run.
Creates a workspace (a directory specific to that particular Pipeline) where work can be done on files checked out from source control.