Right click action menu

[FoX57]

  Can someone tell me what this is

 

[FullMetalMachinist]

That button is a new feature introduced by the 1.1 UI overhaul.  Press it to "pin" that context menu open. 

Previously, the behavior of those context windows was that you would right click on a part to open it, and you could alt-right click on multiple parts to have all their context menus open at once.  This is how resource transfer (like pumping fuel between tanks) woks.  Then you would right click an empty spot on the screen to make them go away.

That is still the default behavior now, but that little button overrides the "right click empty space to hide" part. So now, if you press that button, that context menu is "pinned" open and will not close until you un-pin it.