All snakes inherit a pair of genes, one from each parent for every trait. These genes are what makes a snake look like it does. They determine eye color, skin colors, patterns and everything else about the snake. An allele is one member of that pair of inherited genes. If both allele's are the same it produces "normal" phenotypes. If they are competing with one another the dominant allele's traits will win out and mask the recessive allele's traits.
It must be noted that the terms "normal" or "wildtype" does not mean anything except most common genes in that species of snake. It does not mean that variations are abnormal by default.
In the picture of the corn you see that the lighter colored variant is the "normal" type and the one darker colored variant in the middle is the "abnormal". In that particular kernel of corn the dominant allele masked the recessive allele, creating a darker piece. Simply put the competing allele that control color in that one kernel produced a morphological abnormality.