The Meta Tags section is located in the right sidebar when editing a page of content. Expand the Meta Tags accordion and also expand the Basic tags accordion to reveal the fields.
The main two fields to add meta tag content to are the Page Title and the Description. The Page Title is automatically generated with the tokens that are already there, so there's no need to change it except for maybe the Homepage where you would want something better than "Homepage". The Description field is where you would add a short summary of what the page is about.
Related: What are Meta Tags?