By Default, Gmail highlights the searched words in search results. i.e When you do a search, Gmail returns a list of messages matching the search criteria. You can select any of these messages to view, and Gmail automatically highlights the word/words you searched for.
Fig: Gmail turn on or off highlighting
Fig: Gmail turn on or off highlighting
Sometimes, you might feel it’s distracting.  You have the ability to turn OFF/ON the highlighting by clicking on the “Turn off Highlighting” link on the top right column. Clicking it again turns highlighting back immediately without reloading the page.