Offsets are short, lateral branches or shoots which root where they rest on the soil or under its surface and thus serve for propagation.
"Plant in nursery rows" is a term frequently used in the foregoing chapters. A "nursery" is a place where trees, shrubs or other plants are raised and grown for transplanting. The young plants are generally set out in lines or rows, hence the term "plant in nursery rows."
"Division of the Bulbs" is simply dividing clusters into single bulbs and planting them singly instead of in clusters. The same applies to "division of the roots," "division of the crowns," etc.