Complex sentence: A strobogrammatic number is a number that appears the same when rotated 180 degrees, that is, it reads the same right-side up and upside down (e.g., 69,96,1001).
Split into simpler sentences: A strobogrammatic number is a number whose numeral is rotationally symmetric, so that it appears the same when rotated 180 degrees. In other words, the numeral looks the same right-side up and upside down (e.g., 69,96,1001).