The extension JSP identifies Java Server Pages. These files are html files that also contain Java code, a programming language of Sun (today Oracle) and / or special predefined instructions. This code is executed on the web server and allows to design dynamic web pages.
The user does not see the actual JSP file, but gets sent from the server the generated HTML file, which is then processed and displayed by the browser.
These are JSP files asp or php files. The use of JSP files requires a Java Server Pages capable web server.