The article is a bit lengthy and technical, but to try and explain how I understand it: Feeld had inadequate security measures which made it possible for someone to access your data like messages and pics. It is not a data breech where all of your private data was stolen and made publicly available, but rather it was vulnerabilities that someone could have exploited and targeted individual users. It was fixed, but it took several months for Feeld to fix it.
I remember hearing about Feeld from this forum, so I thought I would post this here to warn people.