Taxing things are stressful and difficult. Climbing a mountain is taxing, and so is spending an entire day babysitting an unruly toddler.
Things that are taxing wear you out, either physically or mentally (or both). You may think of physical jobs like construction work as the most taxing kind of employment, until you realize how emotionally taxing it would be to care for sick patients as a nurse, or how mentally taxing you might find working as an accountant. This adjective comes from the verb tax, which means "to put a strain on."