Where can I add a meta description for page?

Modifying your page description improve the visibility of your page by search engines. Here's how you can customize it in your Page settings.

1. Navigate to the Website menu.

2. Click on Pages... link.

3. From the list of your website pages, choose the one you plan to edit and click on the Gear icon to open Page Settings.

4. When you click on the Gear icon, a new window opens up. Here you can edit the Page description the way it will be seen in search results. 

When writing meta descriptions, try to keep it around 150 characters because Google usually truncates snippets that are longer than 160 characters. Include your most important keywords and key phrases but keep it sounding natural, without keyword stuffing.

You can read more about Page settings in this tutorial.