流量密码 varies by different platforms and online communities.

For example on Youtube it might be something like “Shocked face and big red arrows in the thumbnail, something funny must happen every 5 seconds, ask viewers to like and subscribe 1/3 of the way into the video, video should be interrupted by a pet every now and then”.

Meanwhile for Facebook videos, it might be something like “Attractive people doing stupid things outdoors, accompanied by dramatic teen girl yelling”.

On Quora, it might be something like “Asking something with a clearly wrong premise so that everyone comes out to correct you, preferably involving history or politics”.

So when you see some content that doesn’t make sense, but hits all the right spots to generate more impressions/views/likes, you can say “It’s just 流量密码” which means something like “They’re just playing the algorithms to their advantage”.

I’m sure the concept exists in every language by now, but not every language has a succinct phrase that summarizes it easily. It’s much more than just writing clickbait, fishing for interaction, gaming the content pushing algorithm etc. - it summarizes all of the said and unsaid rules content creators follow to be more successful on their online platform.