A placing (called a placement in the US) is the issue of new securities, which are sold directly to holders, usually institutions. Unlike a rights issue a placing of shares is not an offer to existing shareholders; simply to any suitable buyers who can be found.
The advantage of a placing is that it is a cheap and simple method of raising money. It does not require the paper work and administrative overhead that a rights issue or an open offer does. The shares (or other securities) are simply issued to a small number of new shareholders who are willing to buy substantial amounts of the new shares.
Placings can be unfair to existing shareholders by allowing new shareholders to buy shares at a discount to the market price. There are regulatory restrictions on placings that are designed to protect the rights of existing shareholders. However, they are a cheap, fast and simple way of raising money.