Packer is usually short for runtime packers. It is also known as self-extracting archives, which is software that unpacks itself in memory when the “packed file” is executed. Finally, this technique is also called executable compression.
This type of compression was invented to make files smaller so that users wouldn’t have to unpack them manually before they could be executed. However, given the current size of portable media and faster Internet speeds, the need for smaller files is not that urgent anymore. So when you see some packers being used nowadays, it is almost always for malicious purposes: to make reverse engineering more difficult, with the added benefit of a smaller footprint on the infected machine. For more information about this subject, have a look at the blog post Explained: Packer, Crypter, and Protector.