Background on Numerical Relativity - GRTLCollaboration/engrenage GitHub Wiki
The purpose of Numerical Relativity is to solve the Einstein Equation - in practise this means we want to find the value of the metric (specifically its 10 components and their time derivatives) at every event in the spacetime of interest. The metric tells us about the curvature of the spacetime, and therefore contains information about things like gravitational wave content that we can extract from it having found its values.
In teaching Engrenage I usually assume that students have followed a first undergraduate course in general relativity and have a good knowledge of vectors and tensors (although most aren't so confident in using them in pseudo-Riemannian geometries - ie, curved spacetimes). If you are new to GR I personally like the books by Schutz and Carroll.
Key resources to learn the basics of numerical relativity are provided in Useful Resources , and if you are following this course independently I would recommend reading the introductory Baumgarte and Shapiro book that is listed there. It is a really easy to read and pedagogical introduction that comprehensively covers the key points you need to understand before starting to use the code. This can roughly be divided into two parts: