“a11y” stands for “accessibility.” It is a numeronym, with 11 representing the count of letters between the letter a and the letter y. Other numeronyms you may be familiar with include: i18n (internationalization), P2P (peer to peer), WWII (World War 2), etc.
The word accessibility has different meanings in different contexts. On the internet, the use of the term a11y helps to identify content related specifically to digital accessibility.
I haven't heard of i18n either, used in the example, but I guess its adding non-english language to the internet.
It is a numeronym, with 11 representing the count of letters between the letter a and the letter y