A content type can have multiple fields (except the Enumeration content type as it has multiple values). Each field has a label, an id (API id) and a type. Also you can set the position of the field which will be used for ordering the fields on the content editor page. You can also set a Group name for a field. Fields with the same group name will be displayed on the same tab on the content editor page.
You can also add validations to the fields: Every field can be marked as required and based on the the type of the field you can select from different validations. For example a Single line text input can have a minimum length, maximum length and length range validations.