The aspects from one birth chart to another show how the two individuals get along. Planets making harmonious aspects mean that those dimensions represented by each planet blend together with pleasing results. Planets making inharmonious (often called challenging or dynamic) aspects produce discordant, irritating and unpleasant results.
For example, a man’s Moon blending harmoniously (conjunct) with a woman’s Venus will make for a tender, emotional, sweet interaction between them (if the rest of the synastric analysis concurs). On the other hand, if his Moon makes a challenging aspect (square) to her Venus, then her way of projecting herself as a woman (Venus) will emotionally (Moon) irritate him instead of arousing tenderness.
Now, there is always a mixture of harmonious and challenging synastry aspects between two charts. Don’t expect to find perfect synastry. That probably does not exist. We are simply looking for the positive aspects to be stronger than the negative aspects. If the negative aspects are stronger than the positive, the two people will bug each other so much that the irritation will outweigh the power of attraction between them.