Securities are the same things as bonds.
In fact, the world is a bit more complicated than this but this will give you a good enough understanding. Every day there are people and businesses, including banks, buying billions of dollars of these securities while there are others selling. A bank may want to sell some of its securities because it thinks that it has a better use for the funds. Maybe there is an opportunity to make a loan on a building that is going to pay a higher interest rate than the U.S. government security pays. The bank would call a broker and say it wants to sell these securities.