The Endowment practice is to have a large amount of HYP on an address and only use the interest generated by staking, leaving the principal sum untouched. That way, the address never gets depleted, being essentially self-reliant. As an added benefit, the said principal sum continuously secures the network.
One example of endowment is the bounty wallet ("staking bounty"). Another one is the faucet.
The word comes from endowment funds, which work that way - principal sum shall be untouched, whilst the product of it drives the projects.