When you launch the Scale function GIMP displays the dimensions of the original layer in pixels. You can set the Width and the Height you want to give to your layer by adding or removing pixels. If the adjacent Chain icon is intact, width and height keep their proportion. If you break it by clicking on it, you can set them separately: this will result in deforming the layer.

However you do not have to set dimensions in pixels, you can choose different units from the drop down menu. If you choose percent as units, you can set the layer size relatively to its original size. You can also use physical units like inches or millimeters. However if you do that, you should keep attention to the X/Y resolution of the image.

If you are enlarging a layer, missing pixels are calculated by interpolation but no new detail is added. The more enlarged the layer is, the more blurred it becomes. The exact result of enlarging depends on the chosen interpolation method. You can improve the result by using the filter Sharpen (after scaling) but the best method is to use a high resolution when scanning, taking digital photographs or producing digital images by other means. Not scaling up well is an inherit nature of raster images.