How much low analog voltage can Arduino uno read?
The Arduino Uno uses a processor containing a 10 bit ADC.  This means the ADC can resolve (if we do not consider noise) 2 to the 10th or 1024 voltage levels between zero and the voltage supplied to the Arduino Uno at the AREF pin.  There is a good explanation of the AREF pin in this stackexchange.com question / answer.  It is common practice to supply the AREF with voltages lower than the positive rail the processor is operating at to improve resolution of expected lower voltages.  For example, if you do not expect to measure voltages higher than 3.3 volts on a 5 volt Arduino, use the 3.3 volt supply as a reference.  Relative to using a 5 volt reference, using a 3.3 volt reference decreases your granularity from 0.00489 to 0.00322 volts per step.

Of course there is a lot of noise in most ADC circuits.  Finding the average of many ADC readings helps mitigate this problem.  Consider using exponential averaging.