LTV refers to the percentage of your collateral value that you may borrow against.
Example: If you borrow $5,000 and deposit Bitcoin worth $10,000, your LTV ratio would be 50%. However, if the price of Bitcoin were to go down, your collateral would be worth less, which would cause your LTV ratio to go up.
Abra has different thresholds for the LTV ratio. If your LTV gets too high, you may need to add more collateral to return your loan to its original LTV ratio.