Gardening

Ok, this is a rough science and likely very inaccurate with the pictured device.

Different nutrients in soil can alter electrical signals passed between two or three different probes that are on these kinds of devices. That can be measured and approximate nutrient content can be calculated.

Proper nutrient testing summary: https://www.agrifarming.in/importance-of-nutrient-sensors-in-agriculture-advantages-and-working-principle

Since the pictured device is likely under $5, I would speculate that it has you wet the soil first before the probe is inserted. Nutrient salts can change the conductivity of water and it is trivial to amplify any change in resistance or capacitance on one of those types of needle meters. It only takes a tiny amount of current. The probes themselves could act as a battery of sorts, similar to how you can make a simple battery with a potato. (I would guess we are around 1mA or less with this setup.)

It's not a great solution, it's prone to false readings but its approximate enough for the home gardener. At the very least, you know the soil conductivity: More conductivity is sometimes better than less.

Edit: Looking at that picture, the soil doesn't look very wet to me. It could explain the "lack of nutrients".