This one is easy. Go to System preferences and find Icon Notifications (top line right icon on my system). Someone is sending you advert Notifications, so check each program there and what they are allowed to do.
It can also be Safari notification in case none of these applications looks like source. For that go to Safari, Preferences, and in Websites, there is item (left side lower) called Notifications and it allows web sits to push notifications (aka adverts, aka garbage) to you.
You had to agree to adding any of these notifications - either web site asked or program during installation asked. Keep an eye on these requests and default inclination should be "deny"...