<mark> element is new in HTML5 and is used to mark or highlight text in a document "due to its relevance in another context".1
The most common example would be in the results of a search were the user has entered a search query and results are shown highlighting the desired query.
<p>Here is some content from an article that contains the <mark>searched query</mark> that we are looking for. Highlighting the text will make it easier for the user to find what they are looking for.</p>
A common standard formatting is black text on a yellow background, but this can be changed with CSS.