Complex sentence: A biclique attack is a meet-in-the-middle (hereafter called MITM) attack, which utilizes a biclique structure to extend the number of possibly attacked rounds by the MITM attack.
Split into simpler sentences: A biclique attack is a meet-in-the-middle (hereafter called MITM) attack that also utilizes a biclique. By utilizing a biclique structure, it is possible to extend the possible number of attacked rounds by the MITM attack.