Comment by WillMcCall

The lines represent a connection or interaction between different parts of the codebase. Most often these are dependencies like you mentioned, but a "dependency" could be a parent-child relationship, API call, imported function, and there are certain exceptions such as user interactions. Right now immediate edge labels are displayed on node hover, but I agree that the criteria for inclusion/exclusion of lines should be precisely defined. Any thoughts on what you'd like to see?