If you go to HOME  >  INBOX you will see all your approval messages or workflow actions, typically in reverse date order (most recent at the top). From here approver can:

1. Select multiple rows by selecting a row and then use Cntrl+click or Shift+click to select multiple rows.
2. Click on the mass approval action button annotated above to do a mass approval or rejection. Note: clicking the mass action button will perform the default action (e.g. approve) with other actions (e.g. reject) available from the button menu