title: What does "Box<Fn() + Send + 'static>" mean in rust?
body: <p>What does <code>Box&lt;Fn() + Send + 'static&gt;</code> mean in rust?</p>
  
  <p>I stumbled upon this syntax while reading advanced types chapter. <code>Send</code> is a trait but what does it mean to <code>+</code> a lifetime to a trait (<code>'static</code> in this case) in type parametrization ? Also what is <code>Fn()</code> ?</p>