You need to keep the UI responsive, so you hand off all resource or cpu heavy work off the UI thread, give back control to rendering and responding to user inpu

You need to keep the UI responsive, so you hand off all resource or cpu heavy work off the UI thread, give back control to rendering and responding to user input ... and later need to collect the results in the UI thread to change the UI state. It is about doing things in parallel. There are many ways on how to achieve that, I am just nit picking on performance.