Sodium plus Water (Na plus H2O)

Sodium (Na) reacts with water (H2O) to form Sodium Hydroxide (NaOH) and Hydrogen Gas (H2)

Unbalanced: Na + H2O --> NaOH + H2
Balanced: 2 Na + 2 H2O --> 2 NaOH + H2

This is a Single Displacement reaction.
​The Na is taking the place of ONE of the H's in H2O.