Dynamo Player
Short answer
A simplified Dynamo runner inside Revit/Civil 3D that lets non-Dynamo users run vetted .dyn graphs by clicking a button.
Dynamo Player is the production-friendly face of Dynamo. It hides the node graph and exposes a graph's inputs as a simple form, so end users can run pre-built automations without ever opening the Dynamo editor. Most BIM managers use Player to deploy approved scripts to the team — graph authors mark inputs as 'Is Input' and outputs as 'Is Output', then drop the .dyn in the Player folder.