Complex sentence: MI6 (Military Intelligence section 6) is the British Secret Intelligence Service often known by its initials, SIS with a remit to conduct espionage activities overseas, as opposed to the MI5 which is tasked with internal security within the United Kingdom.
Split into simpler sentences: MI6 (Military Intelligence section 6) is the British Secret Intelligence Service often known by its initials, SIS is one of the British secret service agencies. The SIS has a remit to conduct espionage activities overseas, as opposed to the MI5 which is tasked with internal security within the United Kingdom.