A collateral lock is applied to the accounts that are being held as security. While you will still be able to buy & sell instruments within these accounts you will not be able to transfer or withdraw cash to the value of the lock from these accounts.
The term collateral refers to an asset that a lender accepts as security for a loan. The collateral acts as a form of protection for the lender. That is, if the borrower defaults on their loan payments, the lender can seize the collateral and sell it to recoup some or all of its losses.
Securities, refer specifically to financial assets (such as stock shares) that are used as collateral.