No, it's not to do work in a background thread - it's to do work on the UI thread. So it's normally called from a background thread, in order to manipulate the UI, which can only be done on the UI thread.
The body of the lambda expression is the code which you want to execute in the UI thread.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…