When you create a new campaign, you will see an option "Traffic Optimization".
This feature helps advertiser bid traffic that has high quality and will optimize your campaign performance in the long term.
Once the traffic optimization is enabled, the campaign will only bid the traffic that meet the condition. It will reduce the traffic volume that you are able to bid.
If you are looking for high quality traffic, we would suggest you to enable this feature. However, if you are looking for large volume of traffic, it is suggested to disable this feature and optimize manually.