An SEO reseller is a business or specialist who helps other marketing agencies with SEO-related tasks so that the agency can offer their clients a package which includes search engine optimization even if they don’t have their own in-house SEO team.
SEO is a broad field of study so the exact nature of the tasks they carry out will differ from company to company, but it always involves helping to optimize a webpage for search engines so that it can rank higher in the SERPs for the chosen keyword/keywords.
This may involve building backlinks, creating content, finding keywords, optimizing site structure, creating appropriate image tags, amending existing content, or lots of other possible strategies.
A practical example of an SEO reseller may help you to understand what they do. Whitelabelers Ltd is a SEO reselling website, so you can get a better picture of exactly what they do if you browse their website.
I hope this helps!