What does sailing large mean?

Answer 1

You may mean "By and Large" - meaning steering a course as far downwind as possible, keeping the sails full and the boat speed up.

The answer above is completely wrong. One cannot sail both by and large at the same time since they mean opposite things.

Sailing by the wind (i.e., sailing by) means sailing as close to the wind as possible. In other words, sailing into the wind.

Sailing large means sailing before the wind. That is, sailing with the wind blowing from the aft quarter.

So it is impossible to sail both by and large at the same time since that would require sailing in two opposite directions at once.