Between a stimulus and our reaction/response is a brief moment in which we get to decide whether we will allow our response to be automatic, that is through our limbic system, or whether the executive functions of the brain will intercede and interrupt the reaction to be a more measured and reasoned response.  This brief moment is so short that many do not recognize it and therefore cannot consciously intercede with reasoned thought or measured action, but it is there nonetheless.  The objective of consciousness is to insert itself into this space so that our reactions become less automatic and more measured and controlled.  While this is hard to do, it is no where near impossible, but take knowledge and conditioning to leverage.  But once managed and even controlled our responses can be more measured, conscious and effective in advancing our goals and their achievement.

Human freedom involves our capacity to pause between stimulus and response and, in that pause, to choose the one response toward which we wish to throw our weight.  Rollo May

I read the paragraph over and over again. It basically contained the simple idea that there is a gap or a space between stimulus and response, and that the key to both our growth and happiness is how we use that space.  Stephen Covey

Between stimulus and response there is a space.  In that space is our power to choose our response.  In our response lies our growth and our freedom.  Victor Frankl