Groovy Server Pages (or GSP) is the default presentation technology for rendering views in Grails. It is very similar to JSP where you can easily combine static and dynamic contents in your output. The Grails framework have made it very easy to use yet powerful enough for most use cases.
Before going through your journey in learning Grails, it is important to understand the MVC Pattern. Most web application framework uses this architecture to help make clean and manageable projects. It is surprisingly simple and easy to learn too.