This is down to how the content has been structured by your developer.s What this means is that if on a piece of content such as a page you have been given a property called introduction, summary or abstract episerver find will automatically map this to the summary for use in these weightings.
This is a concatenated string based on all searchable properties as strings on you content item, please see https://world.episerver.com/documentation/developer-guides/find/NET-Client-API/searching/Unified-search/. You can override this behavior by adding a property called SearchSummary on you content item, or by removing the default implementation in your search conventions and then add your own extension method called SearchSummary().
By Default Summary is not populated in EPiFind unless you add SearchSummary property in Page. So if you have not added any SearchSummary field in your page then then summary property in Find->Configure will not have any effect.
In Find > Configure, what does the Summary property read on a page in Epi? Is it the Page description field on the SEO tab?
In the Epi Find User Guide, it just describes the Summary property as "The introduction, summary, abstract, or similar property type." Is that the first paragraph of content on a page?
I'm using Epi as a content editor, not a developer, so the simpler the response, the better for me :)