How to calculate concentrations and what is the concept of 'mole'?
Concentration of a liquid can be given by the formula C=N/V, where V is the volume of the solution and N is the number of moles present in it. 

To help you understand exactly what a 'mole' is, think of it like this: when you ad a certain amount of powder to water, say sugar or a fruit drink, how many spoons/ grams of powder you add is the number of moles. (1 mole is actually 6.02214129 * 10^23 particles of the substance) 

The concentration can be determined by how much water you add to the drink: If you add more water, the solution is said to be diluted, since the ratio of solute:solvent is lower. So in a drink with 30% concentration, the ratio would be 3:7, 3 moles of powder for 7 moles of water. The concentration can ONLY BE CHANGED if the amount of water changes. The ratio remains same throughout, so concentration would be 30% if you take 20ml, 10ml, or any measurement of volume.

Hope this train of thought helped your mind to get the reasoning :)