The pKa is a useful way to express the relative strengths of weak acids (and bases). Note that the lower an acid’s pKathe more ‘acidic’ it is. The Henderson Hasselbalch equation tells us that when the ratio of conjugate base to acid is 1 the pH of the buffer equals the pKa (because the log of 1 is zero). As the ratio of conjugate base to acid increases the buffer becomes more alkaline–when the ratio is 10, the buffer has a pH 1 unit higher (log of 10 is 1) than the pKa of the acid and is no longer a buffer. If the ratio of conjugate base to acid is 0.1, the buffer has a pH 1 unit lower than the pKa and is also no longer an effective buffer. The equation tells us therefore that buffers are no longer effective one pH unit above or below the pKa of the weak acid used to prepare the buffer.
The Khan Academy has a good video on the Henderson Hasselbalch equation and how to use it. You might want to search for some others, particularly if you plan to study biochemistry. Biochemists are always preparing buffers and use this equation to do their calculations.
Derive the Henderson-Hasselbalch equation