Looks like devnet refers to your local/private environment.
The Hydra-POC docs refer to it as 'a single Cardano node producing blocks used as a local devnet'
More details about Hydra-POC set up here:
This is how you run hydra-node in Devnet. You need to have Docker installed.
Keep in mind that, at the moment, this is just a POC:
git clone https://github.com/input-output-hk/hydra-poc