The strength of an acid is usually measured by the amount of protons (H+ ions) that it releases to the solution. The stronger the acid, the greater the tendency to release protons in the solution (thus, higher proton concentration). This is usually given as a measure of pH, which is simply the negative logarithm of the H+ concentration.
Formic acid has a pKa of 3.75; acetic acid has a pKa of 4.76. Which is the stronger acid? Does the stronger acid have a greater or lesser tendency to lose its proton than the weaker acid?