Represents the Process Steps created for particular approval process(ProcessDefinition). This object is used to read the description of process step. In simple words ProcessNode records describes a step in a process definition. We can describe, query, search and retrieve the approval processes Steps.
~ Query ~
SELECT Description,DeveloperName,Name,ProcessDefinitionId,SystemModstamp ,Id,FROM ProcessNode
As we can see ProcessDefinitionId field is acting like a foreign key which is referring ProcessDefinition Object or Table for which steps or process nodes are created. This object is also read only as ProcessDefinition Object.