If you click on a threat that was mitigated, you see that the threat was detected, alerted on, killed (stop execution) and quarantined (prevent further execution temporarily). Since the threat was mitigated, one can be confident that this was a real threat. However, to validate this, you can follow the same investigation steps as provided in Detected Threats.
If you have confirmed that the threat is real, then it is good practice to remediate (delete or remove) the threat from the endpoint. This can be done by clicking on the Remediate under the Actions section.