Short answer: pH = −log₁₀[H⁺]. A concentration of 1 × 10⁻³ mol/L is pH 3, with pOH 11 and [OH⁻] = 1 × 10⁻¹¹ mol/L — an acidic solution.
The log scale, in one paragraph
Because pH is a negative logarithm, one unit is a factor of ten in concentration. pH 5 is ten times more acidic than pH 6, and a hundred times more than pH 7. That is why small-looking pH differences matter so much in water chemistry and biology.
Worked example
Lemon juice at [H⁺] = 5 × 10⁻³: pH = −log₁₀(0.005) = 2.30, pOH = 11.70, [OH⁻] = 2 × 10⁻¹². Going the other way, pH 8.5 gives [H⁺] = 3.16 × 10⁻⁹ and a basic solution.
Assumption. Everything here is at 25 °C, where Kw = 1 × 10⁻¹⁴. Activity coefficients are ignored, so very concentrated solutions will differ from a pH meter reading.