For strong acids:
Strong acids break apart completely in water, so the number of free hydrogen ions is the same as the concentration of the acid. The more concentrated the acid, the lower (more acidic) the pH.
For strong bases:
Strong bases also break apart completely in water, so the number of hydroxide ions is the same as the concentration of the base. The more concentrated the base, the higher (more basic) the pH.
In short:
- Strong acids give lots of hydrogen ions → low pH.
- Strong bases give lots of hydroxide ions → high pH.