The phrase "magnetic pins" refers to small pins or clips that have a magnet attached to them. These pins can stick to metallic surfaces, like a refrigerator or a magnet board, making them useful for holding papers or notes in place. Full definition
Where the Essential Phone differs is that there are a couple of magnetic pins on the back for where you can attach modules like the charging base or 360 - degree camera. (pocket-lint.com)
Essential has also packed a pair of magnetic pins on the back for snapping in specific modules made for the phone, like the 360 - degree camera. (androidheadlines.com)
The proprietary magnetic pin connector is ugly, and the magnet isn't quite strong enough to keep the two pieces connected. (androidcentral.com)