SLA Behaviours define SLA Metric fields' behaviour in response to JIRA issue events. SLA Metric's state is dependent on JIRA issue events. Use SLA Behaviours to set JIRA issue events triggering specific SLA Metric's state changes. Thanks to that you can precisely (and basing on other JIRA events) define when a given action is supposed to be performed.
If you are not familiar with JIRA issue events read more here.