The meridian system is a model developed over thousands of years in the East, on which acupuncture, acupressure, Traditional Chinese Medicine and various other modalities are based.
These Eastern modalities developed in a culture that views the body as a self balancing system, which, when it’s out of balance, requires action to help the body rebalance itself - it's all about pattern rather than the Western idea of pathology or disease.
Now, if you were to ask where the meridians are in the body, I could point you at a diagram. However, although acupuncture is offered for some conditions on the NHS, no one has ever been able to find the meridians. There are no little tubes through which the energy runs, like blood through veins. However, there is research that indicates a measureable change in electric charge on certain points on the body that correspond with the points mapped out on meridian diagrams. There is also research that measures surface conductivity on skin. But Western science doesn't understand the energy model of the body or a system that takes the body/mind/spirit as a whole and therefore tends to dismiss it as fantasy ... until nothing else will work.
Meridians are the energy pathways through and around the body that the Ancient Chinese observed, used and continue to use today.