What is a webpage title?

I believe you are referring to the TITLE of your website as it relates to Search Engine Optimization.

The Title is a part of Meta Data which is the Title of your website. With most platforms today, you can have a Title for each page of your website. The search engine's will "crawl" your site and if you have it set for: index, robots follow the SE will see the Title set for the website and then follow the Index which links to your other pages and then read these titles. This is also done with navigation.