Running with the Land is an important “term of art” in real estate law. It means that all takers of the property after (or in legal speak: subsequent to) the current owner(s) must abide by the terms created by the original owner(s).
These subsequent takers may – and often are – specified as ‘all representatives, heirs, successors in interest, and assigns’ to further assure clarity that the drafter’s invocation of running with the land has its intended meaning.
As an aside, this sort of redundancy is very common in English common law where subsequent to the Battle of Hastings of 1066 – which was won by the French-Normans lead by William II over King Harold Godwinson – legal parlance often included both the English and a Latin or French translation together. 
Returning, Running with the Land language signed on an agreement by real property owners in front of a notary and then recorded with the County Recorder elevates the status of the agreement – from being enforceable only between the individuals signing on the line – to one which is enforceable to all people later taking an interest in the land.
Privity of Contract is the non-elevated case. This simply means as soon as one of the two parties no longer has an interest in the property contemplated within the agreement the agreement becomes non-existent.
In a real property setting, this appears most commonly where there is a Lease. Less commonly, this can also occur where there is permissive use by License.
By contrast, agreements made to Run with Land often include: Easements, Reversionary Interests, and CC&Rs (“Covenants, Conditions, & Restrictions “). 
These tools can often be used to very particularly carve out necessary rights and duties of neighbors to resolve conflict. Thereafter everyone who takes the real property, whether they have actually reviewed the language or not, are held to the terms because they are deemed to have been placed on constructive notice as to any recordings referencing the real property.
Upshot of all this is that one should very carefully consider present needs against potential problems down the road.
Although well-trained, attorney’s are not omniscient and as a result there is no such thing as a bullet-proof contract. Much less bullet-proof is an agreement which is to extend to eternity.
Yet, having an attorney who understands the proverbial ‘lay of the land’ is likely best placed to help you recognize any bend in the road ahead … and help you navigate it.
 Legal Doublet examples include: Aid and Abet, Breaking and Entering, Cease and Desist, Full Faith and Credit, Indemnify and Hold Harmless, Liens and Encumbrances, True and Correct, and Will and Testament.
 Though CC&Rs may appear in just about any recorded document, they are most often found recorded to govern Home Owner’s Associations. Careful legal review of HOA CC&Rs should be done before purchasing any property subject to their terms.