It is not a specific app it is a concept implemented in apps. On Android check Amber for example.
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Ahh ok, I never really looked into it before, only heard about it over the years
I just stumbled on an article about it. nostr:naddr1qq2526thve9xyd23fa9yk4p4dcu4zdmgxdyhqqgswaehxw309asjumn0wvhxcmmv9upzquxy3e954fmcfjfqnecvynugaw97ujy23hznh74lelu5suqvu0xtqvzqqqr4gu8e34gm