Martin 13-56 SP 80/20 Medium

The "13-56" refers to the gauge or thickness of the strings, with the first string being 0.013 inches in diameter and the sixth string being 0.056 inches in diameter. "SP" stands for "Stay Play," which is a type of coating on the strings that helps them last longer and resist corrosion. "80/20" refers to the alloy of the string, which is made up of 80% copper and 20% zinc. Finally, "Medium" refers to the tension or feel of the strings, with medium tension strings providing a balance between ease of playing and tone.