manual:config:triggers:severity

Trigger severity defines how important a trigger is. Zabbix supports the following trigger severities:

SEVERITY	DEFINITION	COLOUR
Not classified	Unknown severity.	Grey
Information	For information purposes.	Light blue
Warning	Be warned.	Yellow
Average	Average problem.	Orange
High	Something important has happened.	Light red
Disaster	Disaster. Financial losses, etc.	Red

The severities are used for:

visual representation of triggers. Different colours for different severities.
audio in global alarms. Different audio for different severities.
user media. Different media (notification channel) for different severities. For example, SMS - high severity, email - other.
limiting actions by conditions against trigger severities