Approval Process is a very amazing feature in Salesforce to automate the business process. An approval process is a set of the steps necessary for a particular record to be approved or rejected by approver or set of approvers.
A step can apply to all records included in the process, or just records that meet certain administrator-defined criteria. An approval process also specifies the actions to take when a record is approved, rejected, recalled, or first submitted for approval.
ProcessDefinition and ProcessNode objects act as a template and store the master configurations for Approval Process itself.