CORS in case of value true will add Access-Control-Allow-Origin=* header
INDEX specifies main index of site (/ page), default is index.html
404 specifies redirect to page in case of unexistent file, default is /404.html
REDIRECT is just total unconditional redirect to other domain (!), for example in case of domain change. Value of REDIRECT acts as prefix and URI is added. For example if actual domain is example.com and value is "https://example2.com" visitor of https://example.com/hello/world will be redirected to https://example2.com/hello/world (in case if you don't need it just put something like https://example2.com/# to REDIRECT and rest part will be almost ignored by browser :) )