What hreflang actually does (and why it breaks)
Hreflang is a hint attribute that tells search engines which locale and language version of a page to show in which region's results. Get it right and your English page shows on google.com, your Chinese version on google.com.hk, your Spanish version on google.com.mx — without duplicate-content cannibalization.
Get it wrong and the worst happens silently. Google ignores the alternate URLs entirely. Your Chinese page shows up on US searchers' queries and gets bounced. Your English page ranks in Hong Kong and tanks engagement. You see traffic fall but cannot trace why.
The most common breakage is incomplete bi-directional tags. If page A declares page B as its Chinese alternate, page B must declare page A as its English alternate. Miss one direction and Google ignores both. Other common mistakes: wrong locale codes (zh-tw vs zh-TW, fil-PH vs tl-PH), missing x-default, hreflang on canonicalized pages, and hreflang URLs that don't match canonical URLs exactly.