To Be Announced - TBA

What Does To Be Announced - TBA Mean?
A term used to describe a forward mortgage-backed securities trade. Pass-through securities issued by Freddie Mac, Fannie Mae and Ginnie Mae trade in the TBA market. The term TBA is derived from the fact that the actual mortgage-backed security that will be delivered to fulfill a TBA trade is not designated at the time the trade is made. The securities are "to be announced" 48 hours prior to the established trade settlement date.

Investopedia explains To Be Announced - TBA
The settlement procedures of mortgage-backed securities TBA trades are established by the Bond Market Association. Each type of agency pass-through security is given a specific trade settlement date for each month. Trade counterparties are required to exchange pool information by 3:00 pm (EST) 48 hours prior to the established settlement date. Trades are allocated in $1 million lots.