What is meant by a mass measurement expressed in this form: 4.6 g ± 0.2 g?

The given mass measurement contains some amount of uncertainty, this uncertainty is 0.2 g. Generally, (4.6 g±0.2 g) does not say that the "true" mass is in the interval 4.4 g to 4.8 g for sure. It means that the mass was measured with a precision of 0.2 g.

Thus, the mass measurement value of 4.6 g ± 0.2 g, describe the range of anywhere from 4.4 grams (4.6 - 0.2) to 4.8 grams (4.6 + 0.2).