By and large means on the whole or in general, taking everything into account rather than dwelling on particular exceptions.
The phrase began as a sailing term. A ship sailing by is close-hauled, heading as near into the wind as it can; a ship sailing large has the wind striking from behind its widest point, a much easier point of sail. A vessel that handled well both by and large could be trusted across a wide range of conditions, and that sense of breadth carried over into the phrase's general meaning.
The idiom summarizes an overall pattern while conceding that exceptions exist, as when a body of evidence is judged by its general shape rather than its outliers. Brian Swichkow's memo 📝By and Large connects the phrase to Ray Dalio's advice in Principles to make decisions at the by-and-large level of understanding rather than on edge cases.
In other words...
Blur your eyes when you look at this set of data