Different coloured objects will reflect different colours of light. Colour is a physical property of light related to its wavelength: red light has longer waves than blue light.
In our eyes are special cells called cone cells that can see different colours of light – we have red, green and blue cone cells that can each pick up on light that colour.
When we see something red, our red cone cells will send a message to the brain telling you that the object is red. When we see something green, the green cone cells work instead.
Other colours are seen as a mixture of red, green and blue. Yellow for example will be seen by red and green cone cells. The mixture of signals to the brain is understood to be yellow. We can’t tell real yellow apart from the right mixture of red and green because of this.
Check out this video which explains in more detail using diagrams: https://www.youtube.com/watch?v=l8_fZPHasdo