What does it mean when my online 90-day report application status shows approval, and are there further steps I need to take?

Your application to file a 90 day report online was approved BUT you're not done yet.

Go back into the system, select the green check status tab, enter either that TM # (by the bar code) or select the personal information tab and enter your passport, DOB and nationality. when you see the approved report click the magnifying glass and view it then select "Next Appointment Print". That will bring up the 90 day report receipt like you get from immigrations and have your next 90 day due date on it. Print that and keep it in your passport