Normally Timeline shows cards from a single board (rows are lists). With Boards enabled, rows are all your boards, a row aggregating cards from all lists in a given board.
For example, if you have 2 boards: Personal and Work, you may switch Boards on to see tasks due “today” both in Personal and Work – on one screen, no matter on which board they are located.
If Boards is off, you will need to switch between Personal and Work to see them – which may be good if you are focusing on Work tasks right now and need them broken down by list in the Work board.