What does the horizontal bar indicate in the DMM display?

That bar at the bottom of the screen simulates an analog meter - the length of the bar increases with increasing digital readings. It is often easier to observe a varying voltage by looking at the bar than trying to interpret rapidly changing digits.

From the Fluke 87V datasheet page 11 point 18 :

The number of segments is relative to the full-scale value of the selected range. In normal operation 0 (zero) is on the left. The polarity indicator at the left of the graph indicates the polarity of the input. The graph does not operate with the capacitance, frequency counter functions, temperature, or peak min max. For more information, see “Bar Graph”. The bar graph also has a zoom function, as described under "Zoom Mode".

The bar is a fast-responding 'analog meter', that can be useful for following variations in the input voltage, for instance when you're adjusting a circuit with a potentiometer in real time.

In the Fluke I had, the digital display updated once per second to 3200 count resolution, while the 32 step bar updated at 25Hz.

This was introduced shortly after DMMs were to counter the criticism that analogue meters were better when you had a varying input and could watch the needle swing in real time. Waiting a second for the digits, and then needing to think what they meant, makes tuning circuits unnecessarily awkward.