Don't let the combination of letters and numbers scare you. The term "a11y" is shortened version of the word "accessibility".
The two middle characters are number ones, not lower-case "L"s. Say it as "A-one-one-Y" or "A-eleven-Y". Pronouncing it as "Ally", while easier to say, is just plain wrong because they're ones, not Ls.
What does "a11y" mean?
The term "a11y" is a numeronym which is a word where a number is used to form an abbreviation.
The most commonly used numeronyms are where pronounciation of the letters and numbers sound similar to the full word. For example "canine" is shortened to the numeronym "K9" (phonetically: "kay" + "nine"). But that's clearly not happening in this case. The phonetics of this word, "ay" + "eleven" + "why" sound nothing like the word "accessibility".
In this case, "a11y" is a numeronym of a different type, where letters between the first and last letters are replaced with a number representing the number of letters omitted.
Similar types of these numeronyms are "internationalization" which is shortened to "i18n", and "localization" which is substituted for "l10n".
Other numeronyms you would know include "W3" for the "World Wide Web" and "9-1-1" for "help".
