Difference between filing date and final action date

I am trying to understand the difference between these two dates...
I filed my I-485 and which date should I be looking at now?

to Cathy:

I’m not 100% sure how they are decided, but I do know that we should reference to filing date this month.

to Cathy:

Since your I-485 is filed, you need to pay attention to the final action date. Your I-485 won't get approved until your final action date is current.
Filing date decides when you can file the I-485.