"realpath_cache_size" is used by PHP to cache the real file system paths of filenames referenced instead of looking them up each time. Every time you perform any of the various file functions or include/require a file and use a relative path, PHP has to look up where that file really exists. PHP caches those values so it doesn’t have to search the current working directory and include_path for the file you are working on.
If your website uses lots of relative path files, think about increasing this value. What value is required can be better estimated after monitoring by how fast the cache fills using realpath_cache_size() after restarting. Its contents can be viewed using realpath_cache_get().