The title text (AKA tooltip or mouseover text) is an HTML attribute Randall puts on almost every xkcd image which serves as Randall's commentary adding something tangentially relevant to the topic of the comic. In some of the early comics, the title text was used to explain the joke (see, for example, 5: Blown apart).
It can be accessed by hovering the mouse pointer over the image on the main site, or by clicking the Alt Text button near the comic title on the mobile site.
Randall uses the 'title' attribute rather than the 'alt' attribute in the HTML sources (cf. Wikipedia's entry on "Alt text").
When using a device that cannot display the title text, it can be found in the offical transcript of the comic.