The term scaling refers to the stretching or squishing of content to fit into a defined area. The "scale" of content refers to the ratio between the hight and width of the content.

For example, most screens today have a scale of 16 x 9.   A resolution of a high definition screen is typically 1920 x 1080 which as a ratio, is 16 x 9. Meaning the width is 16 and the height is 9. This ratio is referred to as aspect ratio.

Older CRT screens have an aspect ratio of 5 x 4. 1024 x 768 was a very common screen resolution for old LCD screens, which as a ratio, is 5 x 4.

However, the content should be created at the same scale as the screen to avoid distortion. Regardless of the resolution, if the content is created at the same aspect ratio as the screen, when the content is scaled both the height and width are scaled equally.   For example:

If a 1024 x 768 (5 x 4) images is displayed on a 1920 x 1080 (16 x 9) screen, the image needs to be horizontally stretched to fill the screen. Scaling content in one direction (width vs height) will cause the content to look distorted and should be avoided. 

The image below shows a 5 x 4 aspect ratio image overplayed on a 16 x 9 aspect ratio screen. A 5 x 4 image would need to be stretched more horizontally then vertically to fill the 16 x 9 space, distorting the content.