Similarity is one type of "grouping rule" that we, as humans, follow in an effort to make sense of our basic sensations and the world around us. We follow these very specific rules unconsciously, but they are very important for our everyday survivial. One grouping principle or rule is "similarity" which refers to the tendency for humans to group together objects or stimuli that seem similar to each other. For example, what do you see here: $$$$$ 88888 !!!!!
Most people would say they see 5 dollar signs followed by 5 number 8's, and 5 exclamation points. But why not see this as 15 items that mean nothing together, or random symbols and numbers? The reason is that we group similar items together to try and make sense of them or find some pattern.