The United States government issues lots of securities and the job of Treasury is to sell those securities and pay them at maturity. Often, Treasury conducts auctions to sell various securities as explained here. In the past, there were actual bonds that would be sold, however today, these bonds have been replaced with bookkeeping entries.
A Treasury Direct Account (“TDA”) is simply an account a company or private party opens at the Treasury to buy and sell securities held in that account. That account is simply the vehicle by which various Treasury securities are held, as explained by 31 C.F.R., part 363: