The collateralization ratio is the ratio between the value of the tokens given as collateral and the value of the Dai debt outstanding, and defined as a percentage.

Any user can generate Dai by locking tokens as collateral. However, because of the price fluctuation of assets, the protocol requires every user to be overcollateralized. The minimum amount of collateral you need to provide when opening an ETH-A Vault is 1.5x of the generated Dai amount. Another way to say this is that the minimum collateralization ratio of ETH to Dai for an ETH-A Vault is 150%. 

Each token accepted as collateral in Oasis has its own minimum collateralization ratio, which can be consulted here.

You have to maintain the collateralization ratio above the minimum required at all times, or the Vault will be liquidated. If the collateralization ratio is dropping to near the minimum, you have two options: 

Repay Dai outstanding: by repaying Dai, you will reduce your outstanding debt. That is, the same amount of collateral will guarantee a smaller debt, and the collateralization ratio will increase; 

Lockup more collateral: this will increase the amount that guarantees the total Dai outstanding, and the collateralization ratio will increase. 

In the example below, 1.5 ETH, valued at $ 882.66 is deposited as collateral to an outstanding debt of 300 Dai. This means that the collateralization ratio is equal to $ 882.66/$ 300 = 294.22% The price of ETH can fall to $ 300 to maintain the minimum ratio of 150%. If the next price of ETH drops and is going below $ 300 (the liquidation price), the user will have to either: repay Dai debt to reduce the amount outstanding or increase the amount of ETH locked as collateral to prevent Vault liquidation.