This vowel is pronounced in two different ways depending on stress. When stressed, it is close to the sound of the English word Ah, but shorter. When unstressed, it is pronounced like u in the word nut.
мама папа вата вода масло
mah-ma pah-pa vah-ta va-dah mahs-la
mother father cotton water butter
You've probably noticed that the letter o is pronounced like unstressed a in the words вода and масло. This is an important rule of the Russian pronunciation so bear it in mind.