One of the most important insights from ecology is that the relationships between things are as important as the things themselves. A healthy vibrant ecosystem is a mass of connections and relationships. That's what we are trying to create with a permaculture system.
Permaculture seeks to integrate elements together so that the needs of one are supplied by another. This is well described by three earlier principles:
Each important function is supported by many elements
For example, food for a household might be provided by a diversity of main crops, wild food, salad and vegetable beds, orchards and soft fruit, agreements with other producers, small and large livestock. If any one source fails, others will provide. This increases the systems 'resilience' and makes failure or disaster less likely.
Each element provides many functions
Many conventional and industrial systems tend to look at 'elements' (e.g. a chicken) as producing only one yield (eggs or meat). This single yield is then promoted and extended often at a cost to the environment and the element itself. In a permaculture system we are trying to utlise all the different functions and yields of an element, e.g. a chicken can provide pest control, tillage, meat, feathers, eggs, heat etc., to increase the overall yields and create a more integrated system.
In order for there to be good connections between different parts of the system, it is important to place them so that they can do this. An obvious example is that a rainwater collection barrel is generally best placed next to the downpipe from a house, or chickens next to annual beds that need occassional tilling and pest control, or herbs next to the back door for easy picking for your next cup of tea!