If the value is greater than 0, she's more helpful and kind. If the value is less than 0, Maria is not terribly pleased with you.
It starts at 0, with the lower limit being -10 and the upper limit at 10. There are various activities and things that you do that can raise or lower the number of points you have and ultimately how much Maria likes you. There is a hidden mechanic that is used for how much Maria likes you.