Depending on the stock or scrip or security you want to place an order with via Dhan, the exchange defines the margin that is required by you to be placed upfront before placing an order. This amount has to be deposited by you in your Dhan trading account.
Every order requires a margin, and the same is visible to you on the order placement screen where Dhan displays both - the margin required and margin available for you to place order. You can check the margin here, or also on the Margins section of the security.