This means that instead of rounding to the nearest whole number, you take the next largest number. So ceil(75.2) would be 76.
However, the spansh fuel calculator seems to behave as though it's using round(fuel) instead of ceil(). Haven't tested the in-game calculation yet but it's gonna be in the noise anyway of course
Edit: I decided to check it. I found that spansh is of course correct and matches the game's calculation - so the fuel consumption is really:
round(5 + (distance * (cargo_weight + tank_weight + 25000)) / 200000)
Surely this bug has been fixed by now though?
[Edit: I just checked and yes it is fixed, yay]