The extension JST identifies JavaServer Pages. These files are html files that contain additional Java code, a Sun programming language (now 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 JST file, but gets sent from the server the generated HTML file, which is then processed and displayed by the browser.
Therein, JST files are similar to asp or php files. The use of JST files requires a JavaServer Pages enabled web server.
A more common extension for JavaServer Pages files is jsp.