Define single genes?
A trait controlled by only one gene.
Assuming that you're referring to single-gene traits, a single-gene trait is a trait that is controlled by only one gene. An example in humans is whether you have a widow's peak - your hairline forming a small point over the middle of your forehead - or not.
The opposite of this is a polygenic trait, or a trait controlled by multiple genes. An example of this is your height - because you have to factor in the length of your legs, your neck, your back, your skull shape, etc.
This can include multiple alleles - or a trait that is controlled by one gene, but has more than 2 possible alleles. An example is blood type - there is an