The Back on Market tool is located within the Listings Menu:

Using the Back on Market tool will change the listing’s status to Active.

All statuses may be placed Back on Market (Back to Active) EXCEPT Sold (and Active, Preview and Hold of course).

When placing a listing Back on Market –

It will post to the Dashboard:

The Days on Market will resume from where it left off.

One example of a common situation for using Back on Market is when an accepted contract is canceled.
Under Contract » Back On Market » Listing is now Active again

If the goal is to reset the Days on Market on a listing, please see this guide:
(MLS) Days On Market (DOM) definition and reset/carry over policy