Explain: Preload and afterload

Per Professor Carl Rothe of IU, the definitions are as follows:

Preload is the end-diastolic volume (EDV) at the beginning of systole. The EDV is directly related to the degree of stretch of the myocardial sarcomeres. This is the basis of the Frank-Starling Law of the Heart.

Afterload is the ventricular pressure at the end of systole (ESP). Ejection stops because the ventricular pressure developed by the myocardial contraction is less than the arterial pressure. This determines the end-systolic volume (ESV). Because the EDV equals the presystolic volume for a given beat of a ventricle, then the pre- and postsystolic volumes define the stroke volume (if the valves are fully functioning and there are no ventricular-septal leaks). The product of stroke volume and heart rate determines the cardiac output—the primary function of the heart.

Hey cfsleo812,
So: really simple, elementary preload and afterload, huh? Good question.
Basically, preload is stretch. The amount of volume being returned to the right side of the heart from systemic circulation. Afterload is squeeze. The amount of resistance the left side of the heart has to overcome in order to eject blood.
Just look at the mechanism of action with different drugs to see what they do. e.g. a dilator will decrease, so the pressure comes down and the heart has less resistance to overcome, the catch is that it can also lead to less venous return, hypotension, ischemia. A pressor will increase, hopefully leading to better return to the heart. But it can also create too much resistance, which also can lead to problems.
This is a very simplified definition, hope it helps. Sometimes the simplest things are the most useful.

Pre load of the right side of the heart is measured by the central venous pressure or CVP

Preload of the left side of the heart is measured by the pulmonary cappilary wedge pressure or PCWP (wedge)

Preload refers to the heart's "filling pressures".