Welcome to Android Central forums.
What device do you have?
How often are you seeing this?
I found this page with a similar icon..
https://www.nextpit.com/status-notification-icons-meaning
About half way down the page under the heading, Stock Android (for Google, Nokia, Motorola, Sony, and Asus smartphones), it says Security Update Good. So if you have one of these models and get it about once a month this could be it.
If not we will need more information.