When scientists talk about waves, they often talk about a wave's frequency. Frequency is the number of times that a wave vibrates in a second. For ocean waves, frequency measures the number of times a passing wave makes a surfer bob up and down in a second. For light waves, frequency measures how many electromagnetic vibrations happen in a second. The frequency of a light wave determines which color light you see.
White light is made up of light waves of many different frequencies.
Two waves can be in the same place at the same time. Suppose two ocean waves of equal size meet. Each wave pushes up and down on the water in its path. Where the waves meet, there are two different forces acting on the water, one from each wave. If both waves push up on the water, the water moves twice as high as it would move if it were pushed by one wave alone.