from Daily Halacha:
3502. Ta’anis Esther is a fast day for men and women. The fast day is to remember that Hashem listens when one is in a time of danger and need, as he did at the time of Purim. The fast is also a segulah to prevent the Satan from prosecuting an inadvertent sin done on Purim. Finally, many Mikubalim write that the decree against us from Haman has a potential recurrence each year and the fast day is a defense against the decree. Shulchan Aruch w/Mishnah Brurah 686:1, Magid Maysharim par. Vayakhel, S’V Shevet Hakahasi 1:203