i have a mastiff that is almost 2, everyone tells me they mature at that age, what does that mean, how do they change? personallty wise?

A puppy is considered a puppy until they are 18 months old. They physically mature at about the age of 2 & they are fully matured at 3. They are just growing up just like any animal would.

Nothing changes drastically... it is all very very gradual and what she is now is what she will be. Mastiff and large / giant breeds don't finally 'finish' as they call it until about 4 yrs old.

But you can say she is mature by 2 yrs and by that you'd consider her an adult.