An ontology is the template for all the actors in your contracts. It defines what information should be collected when an instance of that ontology is used in a case.
A person is a good example for an ontology. Using the word person does not mean a specific individual. It is a generic term, meaning a human being. The ontology of person is therefore a template to describe an individual person. The template could say that every person should have first name, last name and a date of birth. These things that describe an individual person are information of the ontology.
An ontology can also have different roles which other instances can take. An example for an ontology's role could be mother and father which every individual person has. These roles can be taken by other individuals. So a person called Jim Jones could have a mother called Anna Jones and father called Anthony Jones.