Complex sentence: Each 1-pixel-wide vertical column in a particular strip, a "sixel", is then read as binary and encoded into a single 6-bit number, with "on" pixels encoded as a "1".
Split into simpler sentences: Each 1-pixel-wide vertical column in a particular strip forms a single "sixel". Each sixel's pixels are read as binary and encoded into a single 6-bit number, with "on" pixels encoded as a "1".