An acid "donates" H+ ions to the solution, while a base "accepts" H+ ions _or_ donates OH- ions. Both of those actions will decrease the hydrogen ion (H+) concentration, and thus increase pH (-log[H+])
The amino group (NH2) acts as a base by accepting a H+ ions from the solution. It does this by forming a coordiate covalent bond with the unshared pair of electrons belonging to the nitrogen atom. This decreases the hydrogen ion concentration.
Sodium hydroxide (NaOH) decomposes into Na+ and OH-, lowering the hydrogen ion concentration because the hydroxide ion will accept hydrogen ions to form water.