Synesthesia is the practice of using one sense perception to describe another in poetry. This can be a difficult concept for many to grasp. Imagine that a deaf person came to you and asked what a trumpet sounded like. Because he or she cannot hear, you would have no frame of reference to describe the sound to them. However, you could use a sensory image like color to describe the sound a trumpet makes.

    "The red blare of the trumpet sounded"

Above, the color "red" is used to describe sound. A person would probably not choose green or brown to describe the sound "blare". One sense is being used to describe another. While a deaf person may never know the sound a trumpet creates, he or she will be closer to understanding the sound through your description using color. 

What senses are being evoked in the line below?

The soft tinkle of the candlelight, jitters against the wall.

tinkle = sound

light = sight

The light of the candle is being described through the sound image "tinkle".  This blending of senses is a very powerful way of creating concrete images in your work.