A piped link is an internal link that displays text that is different from the title of the page to which the text links. It is created with wikitext that results in hyperlinked (underlined, clickable) text. For example, the wikitext
[[train station|station]] displays as station but links to the train station Wikipedia article.
Do not confuse piped links and redirects: they are two very different mechanisms. Both allow the displayed text of a link to differ from the title of the final destination page (the page that a reader will see after clicking that link). However, a piped link conceals the destination page's title only in that single line on the single page where its wikitext is used, while on any page throughout Wikipedia any link, piped or not, to a specific redirect page will always lead to the same destination page.