What is dead volume ?

The volume of mobile phase required to elute an un retained compound from column which is equal to column volume.
It is very much less possibility if the any impurity elutes before dead volume time, some specialized note shall be required during method development or method validation.
Otherwise method not reliable for evaluation if this impurity. 

what is a difference between drug purity and drug potency?

Potency is a measure of drug activity expressed in terms of
the amount required to produce an effect of given intensity.

Purity is a measure of the amount of API present in a sample
compared to those of related substances, impurities,
residual solvents, etc. 

What is the Difference between Humidity and Relative Humidity ?

humidity is amount of moisture in air in grm per cubic meter
& Relative humidity is % of that compairisan between amount
of vapor present & the amount the air can hold at the given
temp. 

why methanol is better than all other solvents for extraction process

Methanol is more used than ethanol, the boiling point of Ethanol is 78.4 °C while the boiling point of methanol is 64.7 °C. so when you have a methanol extract you need lower temperature to evaporate the solvent in the vapour so the extract is less damaged than the ethanol one.