When a mom and dad both pass down recessive alleles. The only way to get a recessive trait is if the father passes down a recessive allele, which he only could if he either had 2 recessive alleles or 1 recessive allele, and same for the mother. As long as both parents have at least one recessive trait, it can be possibly be passed down.
When a trait is recessive, an individual must have two copies of a recessive allele to express the trait. Recessive alleles are denoted by a lowercase letter (a versus A).
