To create a new page navigate to Pages ▸ Add new. Read more about it in a WordPress Codex article, if you haven't had any experience with this CMS before. You can add post title, upload a post thumbnail (Featured image) and configure other page settings. These are common things, so you need to know more about page templates and how to work with them.
Once you created a page, you can change page template. You can find it in the "Page/Post attributes" section.
The main difference between this template and "Default template" is the ability to hide page content to a "sliding page". It does require a hero slideshow that may represent your portfolio posts, display custom slides or just hold simple featured image. On a regular page content goes to bottom - below the hero header. Showcase template has no page scroll and its main purpose is to display projects/features/advantages etc.
As you can see on the screenshot, there is a small "Introducing" link near the "hamburger" menu. Any page content is hidden by default and can be viewed by clicking that link. You can change link text, usign this option.
This template lists portfolio posts, so it has certain options.
This template can display categories filter ("Filter" link on the screenshot).
You can also modify portfolio design, change layout and filter style at Appearance ▸ Portfolio.
You can use this template if you need a two-column page: hero slideshow on the left and content at the right hand side. Additionally, you are able to use it as contact page, because it has an options bundle to define latitude, longitude and other related settings.
If you are using shortcodes or "Layout Helper" builder for adding content to pages, it's highly recommended to use native "Excerpt" meta box. Text from this field will be visible in search results and used for Open Graph Meta Tags (sharing).