The Quality Gate gives you a Pass or Fail rating for your project depending on the metrics you have provided.
To create a new Quality Gate, go to your Organisation main page and click Quality Gates on the top menu.
Click Create and add the desired conditions with their Warning and Error thresholds. By setting a default Quality Gate, you are assigning every Project without a specific Quality Gate to use the default one. To assign Quality Gates to specific Projects, use the Administration > Quality Gates menu for your Project and select the one you would like to use.