Organizational process assets are one of the most common inputs for PMI processes. The PMBOK glossary describes them as "plans, processes, policies, procedures, and knowledge bases that are specific to and used by the performing organization."
The organizational process assets are described in section 2.1.4 of the PMBOK, 5th edition. They are grouped into two categories: processes and procedures, and corporate knowledge base. The processes and procedures include things like policies, document templates, financial control procedures, and action item tracking. The corporate knowledge base may include things like historical information, lessons learned, various configuration, issue or defect databases, project files from previous projects. The organizational process assets may come from operations as well as projects. One good source for the project-relevant assets is the PMO.
During early phases, it may be useful to review the lessons learned from previous projects, which are part of the organizational process assets. This way the project can benefit from knowledge gained in earlier projects. Some of the PMI processes where lessons learned are specifically called out are develop project charter, develop project management plan, define activities, and plan human resource management.
Organizational process assets are a frequent ITTO.