Unity or proximity referes to the visual linking of the various elements in the design. Based on Gestalt Theories of visual perception and psychology that among other things deals with how the human brain organises visual information into categories or groups. Some of the concepts that might be of use in web design are:
1. Continuance
The general idea is that if you start looking in one direction you will continue to do so until something else or rather more significant is going to catch your attention.
2. Proximity, Similarity and Alignment
The brain tends to group elements of similar colour, shape and size and thus form a semantic relationship. The same applies to elements that are positioned close to each other or aligned.
3. Closure
If our perception of an element is that there are some parts that are missing our brain tends to fill in that missing information. The brain will add the necessary information in order to achive closure.