The phrase "to fill in" means to complete something that is missing or incomplete. It can refer to adding information, details, or content to an existing form, document, or text. For example, if you are filling out a job application, you would be asked to provide certain pieces of information such as your name, address and employment history in order to "fill in" the blanks on the form. The phrase can also refer to taking someone's place temporarily, for instance when an employee is absent from work, another person may fill in their position until they return.