Page title is mandatory for human reading and SEO. Using specific and unique keywords in page titles make your pages more descriptive. Search engines also use page titles as a major weight to determine the relevance and rank. There fore, you should have unique page title or every page on your site. 

Please go to Admin > content > Page title control the page titles. Administrators can configure patterns for how the title should be structured.

The default title pattern is: [page-title] | [site-name]. For example, if your site name is "Great book for cooks", your About us page will have the title as "About us | Great book for cooks".

Master Drupal in 7 hours - Configure SEO - Page title

Basing on your SEO strategy, you can configure page titles here. But do not spam your titles with alot of repeated keywords. When your pages are displayed as search results, the page titles come up and people may look at them as spam links.