Slim is a template language whose goal is reduce the syntax to the essential parts without becoming cryptic.
The initial design of Slim is what you see on the home page. It started as an exercise to see how much could be removed from a standard html template (<, >, closing tags, etc...). As more people took an interest in Slim, the functionality grew and so did the flexibility of the syntax.
Slim will strive to maintain simplicity, but not everyone's definition of a readable syntax is the same. The docs will show you the options.