Fun Facts to Know and Tell:
When your heart pumps out blood, it travels through the tubes in your bod, basically one long whoosh. It pumps out into arteries, which get smaller and are called arterioles, even smaller capillaries (where it drops off oxygen and picks up waste), then the tubes get bigger - venules, and even bigger, veins. Artery, arteriole, capillary, venule, vein.
On the way out of the heart, the blood has little problem going forth to do its duty, 'cause it got a nice shove from the old ticker. But by the time it gets to the venules and veins, it isn't moving quite as easily, especially if it has to go against gravity to get back to the heart, like it does in the legs.
So, God gave the veins a little help. They've got little stoppers, valves that let blood through but prevent it from flowing backwards.

You might be able to see these on the top of your hand or foot. But that sometimes isn't enough, so the muscle acts as a sort of pump, massaging the blood upward.
If you remain inactive, especially for long periods, the muscle won't be as big a help in massaging that blood and pushing it back up to the heart.

When the blood hangs out in one spot for too long, it starts to "pool." It can create a little clump, called a thrombus. If a piece of that clump breaks off, we call it an embolus. Danger, Will Robinson! Emboli are bad. They suck. Little pieces can break off and continue on their way.
Since veins keep getting bigger as you go toward the heart, the embolus doesn't cause many problems. But when blood gets back to the heart, the heart sends it to the lungs before sending it out to the rest of the body. Normally, the blood goes to the lungs through arteries and arterioles, drops off the waste it picked up in the capillaries (you breathe it out) and picks up oxygen (you breathe it in), then goes back to the heart through venules and veins.
BUT, if there is an embolus, when it gets to the lungs it can wreak havoc. That's when things start getting smaller, the embolus gets stuck. Clogs everything up. And as anyone who has ever had a clogged pipe knows, that is bad, bad, bad.
That's why you so often hear of people dying from a "pulmonary embolism." It probably didn't START there, it is just that it doesn't cause a problem until it gets there.
So, staying off of your feet and not letting the muscles help get that blood moving isn't always a good thing.
If you have had a DVT (deep vein thrombosis), you should always follow the advice your doctor gives you. At some points they'll want you lying flat and not massaging your leg because they don't want the thromus to break off and send an embolus on its way. But if you don't have one - its been cleared up, they may want you up and moving every day so that the blood doesn't pool and cause another thrombus.
Do what your doctor says! DVTs aren't always a big deal, but if you ignore your doctor's advice, the results can be serious.
So, wear the knee-socks. If you doctor says you can walk a lot, walk. If he tells you to rent the
ECV, rent it. Don't follow internet advice, follow the doc's!
And make sure you stay hydrated. When there isn't enough liquid, the blood can get, for lack of a better word, "thicker." Lots of other reasons you need water, too (especially in the heat). But that's as good a one as any!
Now you know...and knowing is half the battle!