When you perform a search, and results are returned, your search terms are highlighted in the Messages tab.
For example, using this query:
(error OR fail*) AND exception | count by _sourceCategory| sort by _count
returns the following results in the Messages tab:
Note that the search terms Error and Exception are highlighted in yellow.
Highlighted search terms are limited to the first 1024 characters per message, in order to provide improved search performance.