Click the Hide/Show button next to Activity to hide or show the Activity window. The history of the individual Contact or the whole Company is shown. This is also the place to set a note or action on a Contact. Click on the selector bar right before the Note field to choose any of the previously setup History Actions for this Contact.
Those can be setup in Server Setup > Contacts > History Actions List. Hit the Add button to a note to the History pane. Click on the man icon to just see the Activity log for one Contact, or the building icon for the Activity log for the whole company.
Save or Revert changes with the corresponding buttons. It is also possible to export the Activity log to a spreadsheet by clicking on the export to spreadsheet icon at the right bottom corner.