Somewhere in the world, my actual grandmothers are reading this and angrily exclaiming that I never write even malformed thank-you notes. DEAR GRANDMOMS: I AM SORRY! YOU ARE WONDERFUL PEOPLE AND THANK YOU FOR EVERYTHING. LOVE reddit.com RANDALL.
Randall is writing a letter to his grandmother on a typewriter, thanking her for taking him and at least one other person on a trip. However, due to a habit he's developed from using a computer for so long, Randall inadvertently litters the letter with chunks of blank space followed by website URLs. As if through muscle memory, Randall periodically attempts to check the latest news by pressing a combination involving the Tab key, typing the URL of a specific website, then pressing a combination using the Tab key again.
On a computer, the Ctrl+Tab keyboard combination usually switches to between browser tabs within a window, such as between two Firefox tabs, while Alt+Tab switches between windows. However, there is only a simple Tab key on an old-fashioned typewriter. Pressing Tab there doesn't switch to another screen, it just moves the platen (the typewriter's cursor, so to speak) to the next tab stop, leaving a wide space before the next typing on the same piece of paper. So the key combination that would satisfy Randall's somewhat hyperactive impulses on a computer is dramatically different on a typewriter, where that key instead causes movement of the platen. So, he hits the tab key, types a URL, and hits the tab key again right in the middle of his letter. It also shows Randall's love of news and information websites.
The title text references that Randall's real grandmothers, upon seeing this comic, might feel bad that he doesn't write to them at all — not even poorly-written letters like in the comic. To remedy this, he writes a brief thank you note to his grandmothers... which also includes one Ctrl/Alt+Tab combination.