suri-ashi

Suri comes from "suru" or "kosuru" which means to rub, chafe or file. Ashi, of course, means leg or more so foot in this case. Thus, think of rubbing the soles of your feet against the earth, tatami or whatever surface your on. In the case when you're doing this barefooted, be sure to raise your toes up (like you were a doing a karate mae-geri) so to avoid stubbing your toes on something.
If you come too far off the ground your movement will become ballistic and ineffective.
Learn this well, practise hard, as it appears often within kata and make your movements, transitions quite powerful and explosive.
Keiko Ikkan!