The lines between blocks in BlockSim diagrams are called connectors. They represent the flow of the process/system and the relationship between the blocks they connect.
The default appearance of the new connectors that you add to a diagram will be determined by the settings in the Diagram Style window.
You can also read this tool tip online at http://www.weibull.com/hotwire/issue150/tooltips150.htm.
Learn more about BlockSim: https://www.reliasoft.com/products/reliability-analysis/blocksim