As a retail client, you receive negative balance protection if adverse market movements result in your account balance falling below $0.00. This means your balance would be automatically reinstated to $0.
We also offer trading tools to manage your risk and help avoid that situation, including stops, guaranteed stops and take profit orders. Please note that basic stop-losses are not guaranteed, and guaranteed stop-losses incur a small fee if triggered.
See the following page on our website which explains these tools further: https://capital.com/understanding-the-importance-of-stop-loss-orders