How can I temporarily soundproof my bedroom window?
If you have single pane glass, which transmits a lot of sound, and heavy curtains (as suggested by BMitch) don't do enough, you could try making a panel from 2" sheathing foam (it comes in 2"x2'x8' pieces for about $17/piece; it may be pink or blue depending on brand).   Cut it to fill the inside of the window frame (you can use a sharp utility knife and a straightedge; make several cuts of increasing depth).    Paint with latex paint to go with your decor and to protect the outward-facing side from UV.   This would be light weight enough to easily put into place and could be slid under a sofa or bed when you don't need it in the window.   

If your window is more than 2' wide, it's a bit more of a project; you'd need to get foam-safe construction glue (it will specifically mention that it's safe for foam) and glue the tounge and groove sides together, and add some thin strips of wood cross-ways (ideally on both sides) to stiffen it.