What is meant by a dominant allele?

Alleles give variation within the phenotype expressed by a particular gene. Let's use the ability to roll the tongue as an example. Individuals can either roll their tongue, or not, and this is controlled by a gene. For this example we will say that rolling the tongue is dominant. Lets say an individual inherits one allele that does allows for tongue rolling (A), and one allele that does not allow tongue rolling (a). This individuals genotype would therefore be (Aa). As can be seen, there is an equal number of (A) and (a). So what would the phenotype be? Well, because (A) is dominant, this would be expressed. Therefore this individual would be able to roll their tongue because the recessive allele (a) is masked, and (A) is expressed because it is dominant.