In the eyes of mammals, we see by converting light into electrical signals. Light comes in different colours (frequencies). Some of our eye cells (cones) don’t care much about the colour or frequency, they just turn on and off at any frequency. Other cells (rods) are sensitive to the frequency, and give a stronger signal for particular colours.
Humans have three types of rods, roughly lining up with red, blue and green. This lets us tell how much each colour is made up of red, blue and green light.
Dogs, and most other mammals except for monkeys and marsupials, have two types of rods. They can see colors, but they are color-blind. That is, some things that look like different colours to us look like the same color to dogs. In particular, they have trouble telling red and green apart.