Sensors sometimes expose their metrics in the form of voltage and a controller can use its internal analog to digital converter to read the data voltage and map it to the datasheet.
For example, the Texas Instruments LM35 temperature sensor works this way. Every 10 millivolts of voltage it generates while sensing, it represents 1 °C in temperature. For instance, if the voltage reading is 235mV, then the ambient temperature is 23.5V. You can read more about LM35 here: http://www.ti.com/lit/ds/symlink/lm35.pdf.
