Title Tags are the text that appears in the clickable link on a SERP listing, as well as in your browser tab.

This is considered the second highest priority place for a pages keywords, and you are allowed to use up to 60 characters before your listing will be truncated(…).

While writing Title Tags you need to keep both the bot & end user in mind as it must be good for indexation purposes, while also looking visually appealing to an end user. 

Typically a website or brand name is also included in a Title Tag, separated from the keywords using a | or a -.

The | is preferred as it uses less pixels & enables more content to be featured on the SERP without being truncated, or cut off.