In Photoshop was does "correct the color, contrast, and density of each image so that all three match" mean?

You might want to consider taking a course in Photoshop. Already enrolled? Let the teacher know you're having trouble.

There's more to explain here than a single answer can provide, but I'll try:

- Color: it can vary between different equipment and lighting conditions. A sheet of paper can have a yellowish tint under a desk lamp, a blue tint near an open window, or even look faintly green under a fluorescent lamp. Correcting these tints is a major part of what Photoshop is designed for.

- Contrast: the difference between the dark and light parts of an image. Like color, contrast can vary between images mainly because a camera cannot see the same contrast range as the human eye.

- Density: not quite sure myself what they're asking for, but I guess it has to do with saturation and/or lightness of particular colors.

Newer versions have various tools to help correct colors, but the most basic way is to use the Info palette (along with the Color Sampler tool) to see the difference between RGB values for various parts of an image, and use Image > Adjustment commands to make changes.