F1 Score is the harmonic mean of Precision and Recall, used when the dataset is unbalanced and we cannot decide whether FP or FN is more important.
$$\text{F1 Score} = \frac{2 \times \text{Precision} \times \text{Recall}}{\text{Precision} + \text{Recall}}$$
Source: Chapter 3, Section 3.4 – F1 Score
Examiners expect the definition and the formula for full marks. Mention that it combines both Precision and Recall into a single measure — that's the key idea. Avoid writing a long paragraph; one sentence + formula is sufficient for 1 mark.