If you know the website you are trying to access is safe then you can run the application. Javelin is just a java plug-in program for the browser. The prompt appears because your browser is configured to show prompts when a Java applet tries to run. If it is a website script, then you should first check the website for problems (malware spreading, Trojan injection, etc) by simply typing the website into Google and see what is said about the website.
