TLA stands for three-letter acronym (or three-letter abbreviation) — a name for the whole family of short forms made of exactly three letters: BTW for 'by the way', IMO for 'in my opinion', FYI, ASAP, CEO, CPU. What makes it fun is that TLA is itself three letters, so the word is an example of the thing it describes; that joke is half the reason the word exists. It grew up among computer people and office workers, who spend their days drowning in these short forms, and it is usually used with a smile or a small complaint: 'this report is full of TLAs' means nobody outside the department can read it. You will meet it in tech writing, email, and online talk rather than in ordinary conversation, and it takes a normal plural, TLAs.