How to remove text from browser title?
This question seems somewhat off-topic as you don't know what this thing is and how to generally change that, independent from Drupal, and I'd recommend to find that out first.

But to take a shortcut from here: that's the so called meta title. And you'll be able to customize it pretty heavily by using Metatag.


  The Metatag module allows you to automatically provide structured
  metadata, aka "meta tags", about a website. In the context of search
  engine optimization, when people refer to meta tags they are usually
  referring to the meta description tag and the meta keywords tag that
  may help improve the rankings and display of a site in search engine
  results. In addition, the module provides support for meta tags (Open
  Graph Protocol from Facebook, Twitter Cards from Twitter) that allow
  control of how content appears when shared on social networks.