Security Groups are a way to control access to Office 365 functionality.
Security Groups are a part of Groups within Office 365. To create and view groups, go to the Admin portal at: https://portal.office.com/adminportal
Then select Groups:
You will see a list of the groups, and note you can filter on security groups:
You can create a security group by select +Add a group:
You can then select the group and then Owners and Members: