What is boost used for? When using Search API, editing an index and changing fields that get indexed, you can set the boost for some of the fields. What is this used for?
Boost column allows you to "boost" search results based on certain criteria. You can boost an entity which will make it have higher relevance. The way results are displayed will be by ranking and setting Boost value will add additional ranking to the field. 

Currently you can only Boost Full Text fields.