Hussein bin Ali is Sharif of Mecca and the ruler of the Hejaz, the Arab region in which the holy cities of Mecca and Medina lie. Hussein is a vassal of the Ottoman Empire but he has no love for the Turkish authorities. He has been engaged in a secret correspondence with Sir Henry McMahon, the British High Commissioner in Egypt. The British hope that Hussein can be encouraged to break with the Ottoman Empire and lead an Arab revolt against the Turks.
Now McMahon writes to Hussein with a direct offer. Subject to certain reservations, Britain is happy to accept the establishment of an Arab state comprising most of Turkish Syria and surrounding areas. With Hussein implicitly to become the ruler of this huge country, this is a most tempting offer.
The territories McMahon is not offering to Hussein are areas around Alexandretta and Mount Lebanon where the population is less solidly Arab or religious minorities are locally dominant. In the case of Mount Lebanon, the British are also deferring to the interests of their French allies, who have long established links with the Maronite Christian population. As for Mesopotamia, McMahon makes clear that Britain will be asserting its own interests there and that it will guarantee the security of the holy places in Palestine. The letter also implies that Britain will retain a guiding role in this new Arab state. But despite all these reservations, the prospect of exchanging his Meccan backwater for a palace in sophisticated Damascus is one that Hussein will find it hard to resist.
image source (Wikipedia; link includes text of letter but also spoilers)