Simultaneous contrast refers to the way in which two different colors or values affect each other when they are placed side by side. In other words, the actual colors or values themselves don't change, but how we perceive them is altered.
Simultaneous contrast was first described by the 19th-century French chemist Michel Eugène Chevreul in his book on color theory, "The Principle of Harmony and Contrast of Colors," published in 1839. In his book, Chevreul studied color and color perception, showing how our brains perceive color and value relationships.
To give an example of simultaneous contrast used in composition, notice how Vincent van Gogh uses bright blues and yellow-oranges in the painting "Cafe Terrace on the Place du Forum, Arles" (below) to create an intense, visual push and pull effect.