The main title is the music, often later recorded on soundtrack albums, that is heard in a film while the opening credits are rolling.
[1] It does not refer to music playing from on-screen sources such as radios, as in the original opening credits sequence in Touch of Evil.
A main title can consist of a tune sung by the leading character over the credits, such as Moon River, sung by Audrey Hepburn in Breakfast at Tiffany's, or the main orchestral theme as written by the composer, such as the famous The Pink Panther Theme.
In the film A Hard Day's Night, the title tune was heard over the opening credits showing The Beatles running from their fans.
In movie musicals, the main title nearly always consists of one or more of the songs from the film, played by the orchestra.