The rent on a piece of a land is equal to the advantage obtained by using that land, as opposed to a marginal (i.e. rent-free) piece of land, for the same purpose and given the same inputs. (Wikipedia)
The idea here is that different pieces of land have different productivity. In the first place, people will use the most productive land they have available for its most productive use. As population grows and demand increases, less productive land will be used, and there will be a premium to be paid for use of the best land. That premium, rent, amounts to the difference in production between the most and least productive land currently in use.
One important corollary is that as more land is brought into productive use, the value of rent increases relative to wages. In fact, you don't need to put the next-most-productive land into use it all, merely establish exclusive rights to it, and rents will increase across the board as the next person will need to use some other less-productive land (with a correspondingly larger productivity difference). cf Enclosure