Tangency edges are the lines that are formed when two adjacent surfaces meet. By default the lines are drawn on you parts, but you can turn them off to make your parts look more realistic.
This image shows tangency edges with black lines along the interface of surfaces.
To turn the lines off, click the Tools pull-down menu and select Application Options.
The Options dialog box appears. Click the Display tab, and in the Interactive Component Appearance section uncheck the Display Edges option. This is also where you can change the color of tangency edges by clicking the black rectangle.
When you click OK, the part appears in the graphics area without visible tangency edges.
Why is this important?
Removing tangency edges can improve the look of your parts. When you show your parts and assemblies to non-technical people this can help them visualize or understand the parts better.