You can edit the description that appears in search engines (meta-description), the title (meta-title) and keywords (meta-keywords) in the administration of Prestashop:
Prestashop 1.3: Preferences - Meta-Tags and click edit, if you want to modify the front page click on index.php
The keywords go in "Meta keywords" and description goes in "Meta description".
Prestashop 1.4: Preferences - SEO
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