You can set several alerts for each queue. When a threshold that you define is reached all online agents receive a notification.

Sevice-Level
Service-level is your goal for how quickly you want to answer a portion of your contacts. It's is defined as the percentage of conversations answered within a predefined amount of time.
For example, an commonly used service-level is "we want to answer 80% of our support calls within 20 seconds".

Note that the service-level you set here is used to calculate the reported SLA (Service-Level Agreement) in your dashboard.

Current Longest Wait in Queue
Current longest wait in a queue describes the maximum time a conversation is allowed to wait in queue before an alert is triggered.

Available Agents
This is minimum number of available agents allowed before an alert is triggered.

Maximum Waiting in Queue
This feautre describes the maximum number of conversations allowed to wait in a queue before an alert is triggered.