pH is the negative base-10 logarithm of the hydrogen ion concentration in moles per litre. The logarithm is there because those concentrations span fourteen orders of magnitude, which is unwieldy to write out.
Each unit is a factor of ten
A solution at pH 3 has ten times the hydrogen ion concentration of one at pH 4, and a hundred times that of pH 5. This is why lemon juice at pH 2 is enormously more acidic than coffee at pH 5, despite the numbers looking close.
The 14 is not a hard boundary
Water self-ionises, and at 25 °C the product [H⁺][OH⁻] equals 1.0 × 10⁻¹⁴, which is where the 14 comes from. It shifts with temperature: at 50 °C neutral water sits at pH 6.63, still neutral but numerically lower. Concentrated strong acids can also give pH values below 0, and strong bases above 14.
Strong versus weak
A strong acid dissociates completely, so 0.1 M HCl gives [H⁺] = 0.1 and pH 1. A weak acid does not: 0.1 M acetic acid gives pH around 2.9, because only about 1% of it ionises. Calculating weak-acid pH requires the acid dissociation constant Ka and usually a quadratic.