A mole is simply a unit of measurement. Units are invented when existing units are inadequate. Chemical reactions often take place at levels where using grams wouldn't make sense, yet using absolute numbers of atoms/molecules/ions would be confusing, too.
A mole is a collection of atoms with a mass equal to the atomic weight in grams.
The number of atoms in a mole is 6.02 x 1023. This works because one amu is 1.66 x 10-24 gram, so it takes 6.02 x 1023 of them to make one gram.
This is one of the most useful concepts in chemistry.
The atomic weight of lithium is 6.941amu. This means that a sample with a mass of 6.941 grams has 6.02 x 1023 atoms of lithium in it.
Always remember that this works for molecules and ions, too.
The number 6.02 x 1023 is known as Avogadros number in honor of the chemist who first proposed a law relating the volume of a gas to the number of gaseous particles it contains.