Remember an allele is a different form of a particular gene. A gene is a stretch of DNA that instructs a cell to make a specific protein. The speicific protein is then responsible for a particular characteristic of the organism.
Proteins are made up of building blocks called AMINO ACIDS. There are 20 of these found naturally in living organisms. DNA is built up from monomers called NUCLEOTIDES. 4 different nucleotides exist, each containing a different base, namely A, C,T or G.
Somehow these 4 nucleotides ("bases") must code for 20 amino acids: