A recessive trait is a trait that needs alleles from BOTH parents in order to show up in the offspring, and is usually portrayed by a lower case letter (a). A dominant trait is a trait that only needs ONE allele from either parent to show up in the offspring, and is usually portrayed by a capital letter (A).
So if the parents had the alleles Aa x aa, then the offspring would be either Aa, Aa, aa, or aa, so they'd have a 50% chance of showing the dominant trait (because there only needs to be ONE capital) and a 50% chance of showing the recessive trait (because there needs to be TWO lower-cases).