phone is showing important display message. what does it mean(?

This means that during the refurbishing process, a non-genuine Apple part was used. In this case, the display is not a genuine Apple part. You can return the iPhone if this is not what you were expecting. Or you can keep the device if you’re okay with this.

https://support.apple.com/en-us/HT210321

Got a pre owned phone last week and it’s showing in my settings “important display message” 

Eek, that is not a good sign. I would suggest you to return your phone within the 30-day period if you decide you do not want to keep it. Assuming you ordered it online, you can send an email to the Koodo Web Store at koodo.webstore@koodomobile.com to start the return process.