The catchup server allow to download the blockchain into an airgapped machine, so that it is possible to run algod on the airgapped machine without access to the Internet.
Online machine: download the blockchain
Create a new folder mkdir data.
Download all blocks from the relevant network. For example, to download from mainnet with genesis mainnet-v1.0 run the following command:
catchupsrv -dir data -download -network mainnet -genesis mainnet-v1.0
Copy the data dir to an airgapped machine using you favorite method.
Offline (airgapped) machine
Run the catchup server on some free port (e.g., 50000) and point it to your data dir:
catchupsrv -dir data -addr localhost:50000
Run algod with following command (where xx is your algorand data directory):