The margin widget provides information about your total Bitcoin deposited on BitMEX, your total unrealised profit and loss, and how much leverage you are utilising.
Your Wallet Balance is the sum of deposited Bitcoin, withdrawn Bitcoin (both completed and pending), and realised profit or loss.
Unrealised PNL is all the unrealised profit or loss coming from your portfolio's open positions. You convert unrealised PNL into realised PNL by closing out a position. Please note that this value is calculated against the Mark price.
Unrealised PNL is added to your Wallet Balance to obtain your Margin Balance.
Position Margin is the amount of margin that is allocated to open positions, it is calculated as the entry value of all the contracts you hold, divided by the selected leverage, plus unrealised profit and loss.
Order Margin is the minimum amount of equity required to retain your open orders. It is calculated as the value of all open orders divided by the selected leverage.
Available Balance is your margin available for new positions. It is calculated as follows: Available Balance = Margin Balance - Order Margin - Position Margin.
The reference page for the margin terms can be found here: Margin term reference