The basic idea is that land is taxed separately from whatever is on the land. By levying a higher tax rate on land than on buildings, a local government can make it relatively cheaper to build, and relatively more expensive to sit and speculate on idle real estate. Landowners are thus encouraged to find the best, most profitable use for their property, ultimately benefiting the whole community. Now, some committed followers of Henry George claim much grander benefits from this arrangement than a casual visitor is likely to notice in Allentown or Harrisburg. True Georgists say the land-value tax is the key to solving problems of suburban sprawl, urban poverty, social inequality and more.
Mainstream economists call that wishful thinking, at best.
Philadelphia comes to mind. Henry George actually seems almost prophetic regarding Philadelphia's current plight. Capital is man-made and therefore mobile, he theorized. Land is not. Taxing land does not, and is therefore a fairer and more efficient way to raise revenue.