title: What is the little padlock symbol near every Visual Studio tab?
body: <p>Sometimes on the right side of any Visual Studio tab there will be a little padlock symbol. What is it?</p>
  
  <p>I thought it was something like source-control, but my solution is not checked in source-control.</p>
  
  <p>EDIT: Thank you for your answers so far, BUT I can edit the files.
  When I am in debug mode it has also this padlock sign, but when I am holding my mouse over the tab I can read "Read-Only".
  Not in debug-mode (normal edit mode, nothing running), there is also the padlock symbol. But when I hold my mouse over the tab there is no readonly text.</p>
  
  <p>Is there an other definition of the padlock sign? Or how can I see the padlock sign while editing my normal source code (and no debugging running)?</p>