Grade 11 taught you to read a pH. Grade 12 asks what the symbol MEANS, because the same move is about to be played on four other quantities. Lowercase p is an operator: it takes the base-ten logarithm of whatever follows and flips the sign. So $\mathrm{pH} = -\log[\mathrm{H^+}]$ — read aloud: pH equals negative log of H-plus. Naming the parts: $[\mathrm{H^+}]$ is the hydrogen-ion concentration in mol/L (the square brackets are read the concentration of), and pH is a pure number — the logarithm ate the units on the way through. The hydroxide side gets the identical treatment: $\mathrm{pOH} = -\log[\mathrm{OH^-}]$, where $[\mathrm{OH^-}]$ is the hydroxide-ion concentration, also in mol/L.

Why two scales chained together? Because water quietly ionizes itself, and the product of the two ion concentrations is fixed: $K_w = [\mathrm{H^+}][\mathrm{OH^-}] = 1.0 \times 10^{-14}$ at 25 °C, where $K_w$ is the ion product of water, a pure number. Run p over both sides — the log of a product is a sum — and the whole of grade 11's see-saw drops out as a theorem: $\mathrm{pH} + \mathrm{pOH} = 14$. It was never a rule to memorise. It is $K_w$ wearing a p.