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.
Another programming language called Java (which required a separate plugin in order to run) became very well known and so Netscape decided to try to cash in on this by renaming the language built into their browser to Javascript. Note that while some Java and Javascript code may appear similar, they are in fact two entirely different languages that serve completely different purposes.
Not to be left behind Internet Explorer was soon updated to support not one but two integrated languages. One was called vbscript(only IE supports) and was based on the BASIC programming language and the other was called Jscript and was very similar to Javascript. In fact if you were very careful what commands you used you could write code that would be able to be processed as Javascript by Netscape Navigator and as Jscript by Internet Explorer.
At the time Netscape Navigator was by far the more popular browser and so later versions of Internet Explorer implemented versions of Jscript that were more and more like Javascript. By the time that Internet Explorer became the dominant browser Javascript had become the accepted standard for writing interactive processing to be run in the web browser.
The importance of this scripting language was too great to leave its future development in the hands of the competing browser developers and so in 1996 Javascript was handed over to an international standards body called ECMA who then became responsible for the subsequent development of the language. As a result of this the language was officially renamed ECMAScript or ECMA-262 but most people still refer to it as Javascript.
