The operating voltage is the voltage where the MPPT will operate. The maximum voltage is where the MPPT will blow up if exceeded.
A solar panel has a maximum voltage(Voc), and a maximum power voltage (Vmp).
You want the Voc total of the string to be lower than the maximum solar input voltage on your inverter/MPPT/charge controller.
You want the Vmp total of the string to fall well within the operating voltage of the solar input on the inverter/MPPT/charge controller.
So lets take this growatt highlighted above:
Maximum: 250VDC
Operating:
12Vbat: 15VDC - 75VDC
24Vbat: 30VDC - 150VDC
48Vbat: 60VDC - 245VDC
Now let's look at an example panel:
255W panel
Voc: 37.8VDC
Vmp: 31.2VDC
If you operate the panel far above or below 31.2VDC, then you will not get its maximum power.
So if you run a 12V battery and only care about the maximum voltage you could string 6 panels together and call it a day. It won't damage your inverter, so that's good enough, right? Maybe not. 6 panels has an ideal voltage of 31.2VDC when outputting its maximum 255W. In a string of 6 panels, you'll find the maximum output around 187VDC. If you run the panels at a lower or higher voltage you'll typically get less power out of the panels than is available. So your 12V system will go to its maximum operating voltage of 75VDC. Nothing is harmed, the batteries will still be appropriately charged, but by operating your panels at a lower voltage than their Vmp you are throwing power away.
So while safety requires only that you make sure your Voc doesn't exceed the maximum input, efficiency requires that you make sure the Vmp falls within the operating voltage.