PA stands for Protection on Grade of UVA. And is the Japanese　measurement of sun protection, which is based on the Persistent Pigment Darkening (PPD) reaction reading at 2-4hours of sun exposure. According to info from the wiki, Persistent Pigment Darkening (PPD) method is a method of measuring UVA protection, similar to the SPF method of measuring UVB light protection.

These are the three levels of protection that are commonly seen:
PA+, PA++ and PA+++

Which means, you’re better off picking a sunscreen with PA+++, which will provide you with the greatest protection.

日焼け止め関連商品
日焼け止め SPF50 PA+++／COOL／ペパーミント 当該製品は、最高値“PA+++”でございます 【特徴１：Cool】 塗るとスーッと清涼感 【特徴２：肌ケア】 日焼けの炎症を抑える成分と保湿を保つための成分、贅沢配合 【特徴３：微香】 汗の匂いを緩和するためにペパーミントのほのかな香り