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

SERVICE-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 often 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
The minimum number of "Available Agents" before an alert is triggered.

MAXIMUM WAITING IN QUEUE
The maximum number of conversations allowed to wait in queue before an alert is triggered.