What is pedal edema? Why do people get it?

Swollen feet. Pedal edema (swollen feet) has any number of causes: venous disease, obesity, lymph problems, liver disease, kidney disease, or heart disease. High blood pressure often goes hand in had with these conditions. Some medicines (calcium channel blockers, thiazolidinediones, eg) can cause swelling too.
See below. Pedal edema or swelling of the ankles or higher is due to salt followed by water retention either due to heart or liver or kidney diseases or problems. It is called dependant edema as it occcurs while standing or sittting for prolonged periods and goes down or dissapears when you lie down.

Related Questions

If I get pedal edema will my soles get puffy too?

Most likely not. The tissues in your soles usually dose not swell much comparing to the rest of the foot.
Yes. Most of the edema will be around the ankle and the top of the foot and toes.