NSLayoutConstraint is a constraint defines a relationship between two of user interface objects that must be satisfied by the constraint-based layout system. Available in iOS 6.0 and later in UIKit and available in OS X v10.7 and later in AppKit.
A constraint defines a relationship between two attributes of user interface objects that must be satisfied by the constraint-based layout system.
The relationship involves a first attribute, a relationship type, and a modified second value formed by multiplying an attribute by a constant factor and then adding another constant factor to it. In other words, constraints look very much like linear equations of the following form:
attribute1 == multipler × attribute2 + constant
The right side of this equation can be considered a modified attribute, so this equation could be described in words as “attribute one must be equal to the modified attribute two.” Note that the constraint-based layout system is free to modify the attributes on either side of the equation as part of satisfying the system of constraints that apply to the objects being laid out.
Constraints are not limited to equality relationships. They can also use greater than or equal (>=) or less than or equal (<=) to describe the relationship between the two attributes. Constraints also have priorities, indicating to the layout system which constraint should take priority when two or more constraints are in conflict. The combination of a set of constraints, along with the set of priorities for applying those constraints, can uniquely describe the relationship between multiple user interface elements in all legal configurations for those elements, allowing the system to dynamically layout the user interface as the size and location of user interface elements change.
Source: Apple IOS Developer Library (NSLayoutConstraint class reference)
