The favicon is the small image that appears next to the page name at the top of your browser. You now have a Favicon folder in your Asset Library, so you can maintain them for all your clients or brands.
The OG tags are "Open Graph" tags that Facebook and LinkedIn use when you (or your followers) share your campaign URL on those networks. Now when people see your campaign promoted in their news feeds, it will have the custom image, title and description that you specify in the Design step of your campaign.
Here's a short tutorial video: