The search bar allows you to quickly search for a song in the collection list or a playlist.
Simply typing text into the search bar will reduce the visible list of songs to those which contain that text in any visible column. Pressing Enter will start playing the top match in the playlist view.
Searching begins instantly when text is entered into the search field. Searching is incremental, that is, as you type each character into the text field, the search is narrowed further. This is useful to find a song where you only remember part of a name, for instance.