QTL: the genome region behind a measurable trait
In short
QTL stands for quantitative trait locus, a region in the genome that contributes to a measurable trait varying in degree. Such traits rarely depend on a single gene, but on several loci at once.
Why such traits exist
Some properties are clearly either-or, like the coarse chemotype at the B-Locus. Many others are gradual: plant height, yield, flowering time, the content of a particular terpene. Such continuous traits are shaped by several genes each contributing a little, together with environmental influence. These loci involved are the QTL.
A QTL is not necessarily the gene itself, but a stretch of genome closely linked to the actual gene. How many QTL shape a trait reveals its genetic architecture: a few loci of large effect, or many of small effect.
How QTL are found
In QTL mapping you cross parents with different expressions, measure the trait in the offspring, and search statistically for DNA markers that go together with high or low values. Where a marker reliably coincides with the trait, a QTL lies. This locates which chromosomes the relevant genes sit on.
Link to cannabis
In cannabis, terpene and cannabinoid contents map as QTL on the nuclear chromosomes, with additive or dominant effects of the loci involved. That is the genetic reason aroma is not the matter of a single parent: it hangs on several nuclear loci that both parents contribute.