The title text is an HTML attribute Randall puts on almost every xkcd image which normally adds 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 (m.xkcd.com).
Strictly speaking, calling this the 'alt text' is incorrect, as Randall uses the 'title' attribute rather than the 'alt' attribute in the HTML sources (cf. Wikipedia's entry on "Alt text"), but Randall himself refers to it as Alt text in 45: Schrodinger and in his page with details about the comic, including Randall's transcript. See more on how to access the title text on a mobile device where hovering is not possible here: Transcript on xkcd.
When using a device that cannot display the title text, it can be found in the Transcript on xkcd.
The title text for 377: Journal 2