It means that if the land is used for other purposes, the most important or primary use must be agriculture. For example, pleasure gardening or keeping a few fruit trees for personal use isn't the principal use for residential land.
Land within the boundaries of a city or town will not qualify if the city is providing general services comparable to those provided to similar properties and if the land is not devoted to agricultural use continuously for the preceding five years.
Land used primarily to raise or keep horses qualifies if the operation is limited to breeding and keeping horses for production, where the by-products are colts or fillies. It involves having a brood of mares and either a stud (stallion) or Artificial insemination (A/I) services. Land used for riding, stabling or keeping horses for recreational purposes does not qualify.
A rollback tax is a penalty for taking the land out of agricultural production. It is called a rollback tax because it recaptures the taxes the owner would have paid had his land been taxed at its market value for each of the years covered by the rollback.
The rollback tax is triggered when the land is changed to non-agricultural use.
The person who has the title to the land when the change in use occurs is responsible.