What exactly is work of <meta charset ="UTF-8">

Answer 1

Simply put, when you declare the "charset" as "UTF-8", you are telling your browser to use the UTF-8 character encoding, which is a method of converting your typed characters into machine-readable code.

Answer 2

Expand what IMOCO said, some characters are not supported to print in the browser by default. For example, Chinese, Japanese, many other languages and special symbols. To make those characters printed correctly we need to define the charset explicitly.
Although it's not always the case, adding this line doesn't hurt anyway.