The Similarity Threshold is the score used to determine the match to a screening. 

When setting a similarity threshold for a screening or Country the results returned for the screening, or any screening using the specified Country, will only contain matches at that similarity threshold or higher. 

The higher the similarity score the closer the match is to the screening performed. For example, a screening executed with a similarity threshold of 75 will only return results that have a similarity score of 75 or higher.