From the documentation:
Deploying Fixlets on the main BigFix server requires downloaded patches and other files from the Internet. You can use the Airgap tool in extraction usage for gathering site contents and in non-extraction usage for downloading files (you can ignore the AirgapResponse file generated in non-extraction usage).
You need to use the BES Download Cacher utility if Airgap is “extraction usage” mode, or if the files are not publicly available (such as those from RedHat).
Read more here: Downloading files in air-gapped environments