Your 404 page is shown to users when a URL/page can't be found (perhaps it's a product you've now deleted, or they mistyped the URL). 404 pages are never indexed by search engines, so there's no SEO value to their title and description. Creating a descriptive title and description for 404 pages makes it clear to human users that the page they were trying to get to can't be found.
If the titles and descriptions being generated by default are not best for your shop you have the option to create a custom template using liquid code.
New to using liquid code? Read our guide here: Plug in SEO Liquid
Edit Our Recommended Templates
To make small customizations to your page title or description you can copy the template that we recommend by default and make adjustments.
To edit your title or description template:
Review the 'Default Title Template' and 'Default Description Template' below
Copy and paste one into either the relevant collection template field in the Plug in SEO app
Update the text and liquid code as desired
Press 'Save and preview'
Scroll down and view the new title or description in the 'Live Search Engine Preview'
Default Title Template:
404: Page Not Found - %% shop.name %%
Default Description Template:
The page you requested can't be found, please visit %% shop.domain %% instead
Example Output:
404: Page Not Found - StockPhotoTees.com
If the page you requested can't be found, please visit stockphototees.com instead