The Activity tab represents all activities within the platform that are recorded only for v2 endpoints.
For Adex, the activity is recorded for Accounts, Pricing in Campaigns, Macros (User Defined) and Lists.

The Activity shows which user performed an action, what is the action and the resource id.

On the right side, you can see the time when an action took place. By hovering it, a more exact time is shown. In addition to the mentioned options, there are also filter and search options.