To know how to recognise and describe two or more things and then be able to compute some estimation as to their similarity.
Visual similarity is often the first thing that is considered. There may be obvious and more subtle ways that items and objects are similar. Things can be similar in many ways and not simply in looks. They can have similar uses, similar weights, they may all be waterproof etc. Sometimes a search for differences may help in the estimation of similarity.
The knowledge named 'estimate similarity' is considered to be fairly important.
In addition, the knowledge is thought to be quite complex and quite difficult to learn, it is almost always learned from experience, and is quite specialised knowledge.
Opinion is that it is fairly justifiable knowledge.