Draw half empty rectangle

Graphics[{EdgeForm[Thick], White, Rectangle[], Black, Rectangle[{0, 0}, {1, 0.5}]}]

Or indeed, if you want to be able to change the filling dynamically, you can wrap a manipulate around it.

Manipulate[
 Graphics[{EdgeForm[Thick], White, Rectangle[], Black, 
   Rectangle[{0, 0}, {1, a}]}], {a, 0, 1, 0.05}]

For fun

Example

DynamicModule[
 {fill},
 fill = .5;
 Panel @ Column[{
    Slider[Dynamic @ fill, {0, 1}],
    Graphics[{
      {FaceForm @ White, EdgeForm @ {Thick, Black}, Rectangle[{0, 0}]},
      {FaceForm @ Black, Rectangle[{0, 0}, {1, Dynamic @ fill}]}
      }]
    },
   Alignment -> Center]
 ]

Output