When the World Wide Web was first created in the early 1990s all web pages were static. When you viewed a web page you saw exactly what the page was set up to show you and there was no way for you to interact with the page.
Being able to interact with a web page - have it do something in response to your actions - required the addition of some form of programming language to "instruct" the page how it should respond to your actions. In order to have it respond immediately without having to reload the web page this language needed to be able to run on the same computer as the browser displaying the page.
At the time there were two browsers that were reasonably popular - Netscape Navigator and Internet Explorer. Netscape was the first to bring out a programming language that would allow web pages to become interactive - they called it Livescript and it was integrated into the browser (meaning that the browser would interpret the commands directly without requiring the code to be compiled and without requiring a plugin to be able to run it).
This meant that anyone using the latest Netscape browser would be able to interact with pages that made use of this language.