Secondary Localization is an additional localization whose keyword field the App Store also indexes in a storefront — giving you extra keyword coverage that most users in that country never see.
Beyond the primary localization, Apple indexes the keyword metadata of one or more secondary localizations in many storefronts. These secondary fields rank for search in that storefront even though the app's visible listing stays in the primary language. That makes them a quiet way to capture additional individual keywords once your primary 100-character field is full.
Two rules govern their use. First, phrases don't combine across localizations — each secondary field's words index on their own, so use them for new single keywords rather than re-creating phrases. Second, only the localizations a given storefront actually indexes will rank; adding a localization the storefront ignores does nothing. The Cross-Localization Explorer exists to show exactly which secondaries each storefront indexes.
Example
In the U.S. storefront, Russian is an indexed secondary localization — so a U.S.-focused app can put overflow English keywords into the Russian keyword field and they'll still rank in the United States. Check the Cross-Localization Explorer to confirm which secondaries that storefront indexes before you fill them.