The Rule of Odds states that framing your subject with 2 surrounding objects (thus creating an odd number of 3) suggests balance and harmony visually. We tend to prefer balance and feel comfortable with these groupings of 3. Groups of 2 or 4 can sometimes create a sense of competition where as the odd groupings tend to balance that a bit. This is a very subjective rule, but it does create balance.
Remember though that odd numbers really just refer to the number 3. Objects of 5 or more create more density than the viewer will perceive and the effect is null at that point. Larger numbers of objects, however can be divided visually into groupings of 3, thus bringing more cohesion to the composition.