This plugin provides a simple way for you to use the Twig templating system within WordPress themes.
This plugin provides a simple way for you to use the Twig templating engine with WordPress themes. A problem with WordPress themes is that it is very easy to create templates that are a mess of PHP and HTML. This isn't good for working with neither does it provide a separation of concerns. The Twig templating engine is a great engine that allows you to separate out getting & manipulating your data and rendering it out. You create your themes in the same way as before, only you render a template instead of using PHP within your page structure.
The Installation section provides the steps to using this plugin with your theme.
In the Other Notes section, there is an easy reference for the different functions/filters and what they can be used for.
For Twig, the Twig documentation is the best resource I can offer. It is extremely well documented.