"AM" stands for "air mass coefficient." (The number following the AM, in this case 1.5, is the actual coefficient) This number is used to describe the solar spectrum after the radiation has travelled through the atmosphere (as solar light travels through the atmosphere, parts of the spectrum are removed - most notably, the UV light in the solar spectrum is almost completely removed by ozone in the atmosphere. Therefore, almost no UV light will reach the surface of the Earth and, consequently, UV light will not ever reach solar cells). The method that 1.5 is calculated is outlined in the paper, but essentially it specifies that they are testing the solar cell where the light from the sun has hit the earth at an angle a defined by 1/1.5 = sin(a) (which basically corresponds to the United States or Europe) and that the solar light has travelled through approximately 1.5 atmospheres before striking the Earth (where one atmosphere is the depth of the atmosphere at the equator).