A repack is usually a cracked release from another source that has been heavily compressed to reduce file size. They often come with a custom installer that manages the decompression process and streamlines the installation process, so you don't have to manually install patches/cracks after installation, and can skip optional content. Trusted repackers like FitGirl will also check for malware/viruses in the original crack, so you can be reasonably confident the repack is safe to install. Having said that, always take your own precautions when installing anything from the internet, i.e. virus scan and only use trusted sources.