Normally, changes to the configuration are enabled immediately by clicking. Since this is not practical for some settings (e.g. network configuration or remote maintenance session), there is a queue.
If it is active, all changes are collected. They can be viewed under System > Queue and rejected or activated together.
It can either be activated manually in System > Queue or it is activated automatically when some settings (network, firewall) are changed.
The queue is shared by all users on the system. If no change is made for a certain time, the queue will be deleted automatically. If the queue is active for a longer period of time, there may be problems when configuring new clients via DHCP.
If changes are made that are not valid due to dependencies alone (e.g. changing a network, if there are still IPs in the old network), these changes are normally invalid and displayed in red as errors. The queue allows such changes, as it is possible to use it to correct the dependencies. If any changes in the queue are to be made, all dependencies must be corrected first.