What is a Jenkins executor?
It is a designation in the Jenkins master server's settings that governs resource allocation (e.g., CPU and RAM) on any Jenkins nodes and the master server itself. When a server (either the master or a node) is designated as having two or more executors, the server can process as many jobs simultaneously as the number of executors for which it has been designated.
In other words if a server has been configured to have three executors, this server can process three jobs concurrently. If the server has been configured to have one or zero executors, then the server can process only one Jenkins job.
The verbatim definition on the Jenkins website of an executor is as follows:
"A slot for execution of work defined by a Pipeline or Project on a Node. A Node may have zero or more Executors configured which corresponds to how many concurrent Projects or Pipelines are able to execute on that Node."
An executor is a subprocess of a main process on a Jenkins node (according to this posting).