The index price is shown on the trading chart and is denoted '.BTCX, INTERDAX' for the bitcoin perpetual contract.
The index price (also known as the mark price) is used as a trigger for liquidation.
An example is shown below:
By default, the index price shown on the chart is the closing price for each trading session. You can change the settings to show the high or the low of the index price as well.
For instance, if you are in a long position and the price is near your stop loss, you might want to change the settings to low, to show the lowest index price that was reached during a trading session.
Click on the settings icon for '.BTCX, INTERDAX' and select 'Low'.
If you are in a short position, you might want to change the settings for the index price (i.e., .BTCX, INTERDAX on the chart) to High instead (shown below).
The index price is also shown in the order book panel, below the last traded price: