Hello,
The sensor measures the stress (which is converted to electrical signals). As Bill Yu points out in his reply, you can measure the digital or the analog signal depending on your intended use. If you simply need the state changes then perhaps digital signals would be best for you. I use it to measure the load and therefore prefer a more continuous signal (with non-linear calibration). One characteristic of the analog signal is that the sensitivity of the sensor makes the output curve very spiky indeed.
There are published datasheets for various piezo-electric elements but I’m not directly familiar with the Grove product.
Kind regards.