A mole, often shortened to mol, is just a large number — approximately 6 x 1023, in other words 6 with 23 zeroes written after it. Conceptually, you can think of it in much the same way that you would use ‘a bajillion’.

So one mole of calcium ions just means there are 6 x 1023 ions, 1 mol/L salt solution means you have put 6 x 1023 molecules of salt into one litre of water.

The only special property of this number is that if you take one mole of Carbon-12 atoms and put them together, they would weigh exactly 12 grams. Because a carbon atom weighs about 12 times as much as a hydrogen atom, one mole of hydrogen therefore weighs about 1 gram.