Probably a PHP microframework like Slim or Lumen. It highly depends on your background and how you define the scope of what you want to be easily understandable

Probably a PHP microframework like Slim or Lumen. It highly depends on your background and how you define the scope of what you want to be easily understandable. I would recommend different systems depending on whether you are a computer scientist, frontend developer, webdesigner, data scientist etc. For most people though it probably is PHP. There is a reason for PHP's popularity and it is its low barrier to entry for beginners. Hosting is plentiful and comes with a standard way to store data (MySQL), deployment is simple and everything being bound to a request (and dieing afterwards) lets you get away with mistakes.