The phrase "by and large" is an idiom that means "generally," "usually," or "all things considered," all of which are used to describe a fact or rule that has few exceptions. Other frequently cited definitions for the idiom include "in all cases" and "in any case," which are interpreted similarly to the other definitions. The origin of the idiom varies somewhat from source to source, but a common idea among all of the interpretations suggests that it came from terms used in sailing.
Since there is so little variation from situation to situation in the definition of this phrase, the usage generally stays the same as well. For example, a company owner may describe his company's good track record by explaining that "By and large, things go exactly according to plan here." Similarly, an individual who lives in a large city might complain that "City traffic is so awful, by and large, that it is impossible to get anywhere on time." In both very different situations, the uses of the idiom were meant to explain that something specific happens the majority of the time.
Occasionally, people misinterpret this idiom to mean that something almost happened instead of that something almost always happens. For example, an excited student in a high school play might say "We filled up the auditorium, by and large." A more common misuse of the idiom occurs when someone uses it to mean "sometimes," as in saying "By and large, we order our products from this distributor, but we also go to the one down the street fairly often."
It is commonly agreed upon that "by and large" describe two sailing situations, and thus the idiom likely stemmed from the use of nautical terms. The nautical terms "by" and "large" respectively describe sailing into the wind and sailing with the wind. One interpretation of the idiom states that, because a ship cannot sail both into the wind and with the wind at the same time, and that when sailing, the ship must be doing one or the other at all times, "by" and "large" represent the only two options possible. Therefore, if something happens both when the ship is sailing "by" as well as "large," it happens most or all of the time. By and large then developed to be interpreted to say "as a general rule" because something that happens by and large happens most or all of the time.