The "Enable LiveSearch" checkbox will start showing visitors the first results as they are typing in the search box.
While this feature is very useful for smaller to medium-sized sites, it can be a performance hazard if you get hundreds of thousands or millions of visitors per day. If your site is very heavily visited, consider turning this off, and measuring the effect on server load.
For enterprise-sized sites, there are ready-made add-ons that will integrate with search solutions like Solr or ElasticSearch.
The other options allow you to specify which content types are to show up in search results, and when to crop the description.
More content types will appear here when you install or create new ones.