As explained in many articles like this one:
Atherogenic dyslipidemia comprises a triad of increased blood concentrations of small, dense low-density lipoprotein (LDL) particles, decreased high-density lipoprotein (HDL) particles, and increased triglycerides
Generally speaking, if you have low HDL cholesterol and high triglycerides, you will likely find a preponderance of small, dense low-density lipoproteins (for us, we consider this 50% or more of your total for LDL-P).
Atherogenic dyslipidemia, as the name suggests, is closely associated with atherosclerosis. Conversely, having high HDL and low TG is associated with longevity reduced heart disease (and is also related to my LCCholesterolChallenge here).
If on low carb and observing these numbers, we usually recommend tackling triglycerides first. Our dedicated page on high TG for low carb can be found here.