A "closed" low is what you likely think of as a low pressure system -- pressure (and height) contours "surround" the low. There really isn't such a thing as an "open" low, though. When a "closed" low opens up, we typically call it an "open" trough. In this case, the isobars do not make a closed loop. In this regard, the terms "open" and "closed" are actually a little redundant. Low pressure systems (and high pressure systems) are closed by definition, while troughs and ridges are open by nature. Of course, lows can have associated troughs too -- e.g., cold fronts associated with a low are troughs.
At the surface, when pressures fall, there's almost always a low nearby associated with the pressure falls (diurnal pressure changes associated with "daytime heating" aside). In other words, we don't usually see a bunch of troughs moving around at the surface (other than those associated with fronts). Aloft, however, the flow tends to be progressively less curved, which means that we tend to see much more in the way of troughs and ridges vs. lows and highs. The terms "open" and "closed" are used most often in reference to mid- and upper-level systems. For example, you may read "the trough deepens tomorrow and cut-offs Saturday evening over Arizona"... In the NH, flow to the north can "pinch off" a high-amplitude trough to yield a "cut-off" or "closed" low. In many cases, such lows tend to more relatively slowly, and some can stay "anchored" in a single place for a week or more. Eventually, such lows usually "open" into a trough and move out of the area.
