Generally, my understanding, is there is a “flow” and a negative reading would be flowing the other direction. Thinking about a multimeter - it usually expects flow from red to black, but if you swap those around, you get a negative number as it flows from black to red.

The actual number is usually (not always) the important part - the potential difference between the two ends or something like that. That difference is going to behave similarly no matter which direction, but we assigned it numbers like that to make talking about voltages easier.