why do digital multimeter show voltage when we touch the probes
There is a good answer (and plenty of nonsense answers) to your question by PaperDoc on AnandTech.


  The moisture on your skin is an electrolyte - it contains a few slightly conductive dissolved materials. AND those materials are a little different between your two hands. So YOU are the battery, and the voltmeter is doing its best to show you the output. Now, the concentration of dissolved materials in each hand may vary a bit over time. ALSO, the resistance of the contact between your skin and the probe will vary over time as your hands flex. For both those reasons the voltmeter will actually "see" a varying voltage, and it will be very small since the source is a Concentration Cell driven by very small differences in the electrolytes (the skin moisture) at each metal probe.