JSP stands for Java Server Pages.
You use it to create dynamically driven websites.
Similar technologies would include asp, php etc.
You need a web server that will host the JSP files.
You need Java installed on the computer so that they can be executed...you need a Java compiler to compile the pages for you.
When the web server receives a request for your page, it will execute the JSP code. That code will generate HTML that is sent back to the browser for it to render the page.
-Frinny