The three weeks between the 17th of Tammuz and 9 B’Av are a time of mourning the destruction of Jerusalem and all the calamities that have befallen the Jewish People throughout history. For those three Shabbatot, the custom is to read a specific set of Haftarot that are called “The Calamitous Three”, in Aramaic, “Tlata de’Puranuta”. The first week we read the first chapter of Yirmiyahu which warns of the impending destruction of Jerusalem.
It usually comes out on Parshat Pinchas. Rarely, it comes out on Matot.
Linear annotated translation of the Haftarah of Matot
It is also the Haftarah read for Shemot by the Sefardim, and is listed in the Rambam as such. The comparisons to Moshe are clear and illuminating.
As to what it teaches us about the Three Weeks, we have : Calamity and Consolation