A more thorough explanation:

Err

Err is a verb that means to make a mistake or be wrong about something.

• The teacher marked my answer wrong because I erred in my calculations.
• The company lost a lot of money because they erred in their business strategy.
• The doctor erred in his diagnosis and prescribed the wrong medication.

These examples show how err means to make a mistake or be incorrect about something. In the first example, the person made a mistake in their calculations and got the answer wrong. In the second example, the company made a mistake in their business strategy and suffered financial losses. In the third example, the doctor made a mistake in their diagnosis and prescribed the wrong medication, which could have had serious consequences for the patient.