Passive advertising is what you think of like an ad in a newspaper. It's there at a cost of hundreds or thousands of dollars, people see it, a small number of people reply, and hopefully enough buy to cover the cost of the ad.
On the other hand, digital Bluetooth billboard advertising is passive advertising too. The difference is that it costs nearly nothing (or nothing) and you only get a headline. You have the opportunity to spark enough interest with 40 characters to get some people to tap the notification and effectively "click" through to your landing page. And that's where you do your selling. The 40 characters sell the "click," the landing page sells your product or service.
The digital Bluetooth billboard advertising on your Gem devices are out there working for you 24/7/365. When people are nearby to your device, checking their phone and notifications, and are interested in your product or service you will get visitors to your linked landing page. Just like newspaper advertising, but for a fraction of the cost and displayed where people check 85 times per day. Nobody checks a newspaper more than once or twice, and it's kitty litter the following day.
Reminder: Each message can be a maximum of 40 characters. Forty characters is not much space, so you need to make those characters count. Think of it this way; if you spotted a notification on your phone for what you are selling, what message would get you to click through to take a look at it?