A/B test

An A/B test is a test where you want to compare two different versions or approaches against each other. For example, a blue colour on a button or a red colour. Then you present these two different versions to the visitors and get an answer as to which colour has performed better. Then you know which colour to put on your buttons. It doesn't have to be colour, it can be anything from text, images, placement, etc. 

You simply put version A against version B and see which one gives the best results. 

For example, think about how you shop at the grocery store. If you haven't decided in advance which brand to buy, chances are you'll buy the one at your eye level or similar.