How does the pKa level correlate with acid strength?

The pKa is a way of measuring the strength of an acid in water solution. An acid, in this context, is a molecule that releases a proton (H+) when in water. The more readily this molecule releases a proton the stronger the acid is. You can think of it as the molecule in question "pushing away" the proton. The lower the pKa value for an acid, the stronger the acid is and the more the (acid) molecule will push away the proton when immersed in water. For example, acetic acid has a pKa of 4.76 (mild acid), phosphoric acid of 2.16 (strong acid) and hydrochloric acid of -7.0 (very strong acid, aka stay away).