A downloader is a program that downloads and installs/runs one or more malware from the Internet
Downloaders are similar in purpose to droppers, and are used as a means of bypassing security checks and getting malware into the target
The difference between a dropper and a downloader
The difference between a dropper and a downloader is that a dropper has the malware inside the program, while a downloader downloads the malware from outside.
So even if malware scanners can figure out how to hide known malware, unknown new downloaders can evade detection by malware scanners.
Downloaders differ from droppers in that downloader techniques are not intended to hide malware, but are more like backdoors for the specific malware downloaded.