What does this equation mean?

where $N_{\mathrm{active}}(t)$ , the number of pixels crossing threshold at a given instant, is normally far smaller than the total pixel count $N_{\mathrm{pixels}}$ for a mostly-static scene. That scene-dependence is also the sensor’s clearest limitation rather than a free efficiency gain: because each pixel’s operating principle is defined by Gallego and colleagues as asynchronously measuring per-pixel brightness change and encoding “the time, location and sign” of that change [ 3 ] , a highly textured, fast-moving, or vibrating scene drives $N_{\mathrm{active}}(t)$ toward $N_{\mathrm{pixels}}$ , and the bandwidth and power advantage over a frame sensor narrows or disappears for exactly the scenes where an application often needs the most from its sensor. The advantage is real and large for a mostly-static scene with occasional motion; it is not a property of the device independent of what it is pointed at.