That most likely means you have installed an App which perhaps you thought might be useful, but its main purpose is to deliver ad popups to you. It happens.
Solution: Find the App in question, uninstall it.
So … how to find the App …
First try: If an ad pops up, swipe down the notification area. The App in question might simply be listed there at that moment. Most Adware Apps hide their presence better though, I guess, so …
Second try: Go to Settings and search for “draw” … with this you should find a “Draw over other Apps” entry (this may differ in other languages, of course), where you can see a list of Apps having that permission … the culprit should be among them (at least this list should be shorter for a start than going through all Apps).
Third try: Go through all your Apps and uninstall everything you don’t really need, one by one, until the ads stop.