In v3, the health of an account is used to determine if an account can open a new position or be liquidated.
There are two types of health:
initial health, used for opening new positions maintenance health, used for liquidations
They are both calculated as a weighted sum of the assets minus the liabilities, but Maintenance health uses slightly larger weights for assets and slightly smaller weights for the liabilities.
Zero is the threshold for both healths. If your Initial health is below falls below zero, you cannot open new positions and if your Maintenance health falls below zero you will be liquidated.