Glare and reflection on the screen can increase eye fatigue or result in you not being able to see parts of the screen at all if severe.
Firstly, it's important to try to eliminate reflection and glare at its source. Use blinds to block out light from windows that is troublesome. Overhead lighting is over diffused with special coverings or bulb types.
Secondly, if feasible move so that the source does not affect you (eg. move the screen so that it is perpendicular to problematic windows).
Use an anti-glare guard on your screen as a last resort.