An orphan page has no internal links pointing to it. It may exist in the sitemap and be technically reachable, but nothing on the site leads there, which signals to both crawlers and readers that it does not matter.
Landing pages are often orphans deliberately — that is correct for single-goal campaign pages.
Everything else should be reachable within three clicks of the homepage.
Audit by crawling the site and comparing discovered URLs against the sitemap.