High pKa refers to a weak acid. The strength of an acid refers to its ability to lose a proton. A strong acid has a low pKa whereas a weak acid has a high pKa.
A strong acid ionizes completely in an aqueous solution. For an acid to be considered strong, it must be stronger in solution than the hydronium ion. A strong acid has a pKa of less than -1.74. Any acid with a pKa greater than this value is considered almost strong or weak, and its ability to donate protons is low. Acids with a high pKa include acetic acid and oxalic acid, which dissociate incompletely in solution.