L2 vs L3 forwarding

Hi all,

I have a doubt about something I learned from a course on INE. In particular, it is said that one of the differences between L2 and L3 forwarding is that in the first case the "forwarding must be symmetric because the MAC address table was learned in this way". I don't understand what it means and I don't understand the difference with forwarding at level 3?

Could someone be so kind as to help me?

Thanks