The Portfolio menu option on the left is where you add Portfolio items. To add a new portfolio item, select the Portfolio > Add New option from the left hand WordPress menu.
You will then be able to set a title for the portfolio item, add the body content, and also provide all the meta information such as extra details and media.
In the side box, categories, skill and feature image
You can add categories for each portfolio item's which act as the filters in the filtering system, and also a way to create multiple portfolio pages. You can have subcategories for filtering on a category Portfolio page, so none of the functionality is lost. When setting up categories, it is important that you name the slugs correctly. The slug of each category should be the same as the name, but all lowercase. If the category name contains spaces, then these should be represented in the slug with a '-'.
You can use the Page Builder to create the content of the item, which effectively allows you to have limitless possibilities for what you can display, and also how you can display it.
On the portfolio Meta box. You can add portfolio description, Link to Item, And the Client