Pledge is creating charge on some asset of the borrower as a security for payment of the loan. For example, if a person borrows a loan from bank against Fixed deposit, the Fixed Deposit Receipt is pledged with the Bank. The bank is at its liberty to use the proceeds of FDR on failure of the borrower to make payment.
In the context of a demat account it will mean creation of pledge on securities lying in the demat account in favour of a lender.