Christian SEO refers to Search Engine Optimization (SEO).
Search Engine Optimization, as defined by Oxford Language, is the process of maximizing the number of visitors to a particular website by ensuring that the site appears high on the list of results returned by a search engine.
However, if I had to provide my own definition of SEO, I’d say:
Search Engine Optimization (SEO) is the process of Optimizing a website to ensure it ranks high on the list of results returned by a search engine, by properly using SEO-Best-Practice techniques.
In layman’s terms, I’d say that SEO is: “Trying to get your website to the top of Google” in order to drive more visitors to your website.
For example, if you were a Christian retail store who sold John 3:16 T-Shirts, you should want to rank at the top of Google every single time anyone in the world searched “john 3 16 t-shirts.”  This would likely help you to sell many more t-shirts.
There are, however, many different optimization techniques that can be used in properly performing Search Engine Optimization.