A woman is told her weight has a standard score (z-score) of - 1.5. this means that

This means that her weight is 1.5 standard deviations below the mean. The z-score informs us how many standard deviations above or below the mean a sample (in this case the woman's weight) is. Standard deviation is a statistical measure that expresses how much a sample differs from the mean value of a population or a group of samples.