You are probably most familiar with search highlighting when you use an Internet search engine such as Google. Most search results come back with a snippet of text from the site containing the word(s) you search for, highlighted. Solr can do the same thing. In the following screenshot, we see Google highlighting a search including Solr and search (in bold):
To conserve screen space, you might even use this feature to simply tell the user that there was a match in certain fields without showing a highlighted value. This could make sense if there are many metadata fields. Nevertheless you would still likely highlight some.