Please look up "Portal" in FileMaker help and any training materials that you have available. This tool is too important not to study and understand thoroughly.
A portal is a tool for displaying and working with multiple related records from the context of a single record in a parent table. So if I have a table of products and a Bill of Materials (BOM) table linked to it. I can pull up a product record on a layout based on the products table and a portal to the BOM table can be used to both see and edit any records in the BOM table that list the materials used to manufacture that product.
It typically, but not necessarily, looks like a small window set to a "table view" where each row in the portal is a record from the related table. If you enable "allow creation of records via this relationship", you can enter data directly into a blank "add row" of the portal to create new related records. In such case, FileMaker copies over match field values from the current record on the layout record to corresponding match fields in the new portal record to link it automatically to your current layout record.