After Taĥanun, and on Mondays and Thursdays following the Torah reading, men regularly recite three prayers. The first is Ashrei (Tehilla Le-David). Although this prayer was already recited in Pesukei De-zimra, it is repeated because of its considerable significance (Peninei Halakha: Prayer, 23:1-2 and ch. 14 n. 5 state that men recite Ashrei thrice daily).
Afterwards, we recite the psalm “La-menatze’aĥ mizmor le-David, ya’ankha Hashem be-yom tzara” (“For the One Who grants victory, a psalm of David. May God answer you on the day of distress”; Tehilim 20), which serves as a continuation of Taĥanun. Since La-menatze’aĥ invokes the day of distress, it is not recited on days of joy (as listed in siddurim; the differences in customs are clarified in Peninei Halakha: Prayer 23:1).
After that, U-va Le-Tziyon, also called Kedusha De-sidra, is recited. It contains the verses “Kadosh, kadosh, kadosh…” (Yeshayahu 6:3), “Barukh kevod Hashem mi-mkomo” (Yeĥezkel 3:12), and “Hashem yimlokh le-olam va’ed” (Shemot 15:18). The uniqueness of this Kedusha is that the verses are recited along with their Aramaic translations. The Sages ordained their recitation (even though they were already recited in Yotzer Or and in Kedusah of Ĥazarat Ha-shatz) so that every person praying would learn some verses of the Prophets every day. The verses are translated into Aramaic, so that the whole nation, which was fluent in Aramaic at that time the prayer was ordained, would understand their meaning.
The reason for instituting the recitation of verses from the Prophets is so that men, who are commanded to study Torah, to learn verses of Torah, the Prophets, and the Sages every day. By reciting Shema they study Torah, by reciting these verses of Kedusha they study the Prophets, and rabbinic teachings are added at the end of the prayer service.
The Sages offer great praise for the recitation of Kedusha De-sidra, stating that after the destruction of the Temple, it became one of the things in whose merit the world stands (Sota 49a). Rashi explains that its recitation possesses two virtues: it is a form of Torah study, and its verses deal with God’s holiness, so when they are recited in a minyan, God’s name is publicly sanctified. There is no need to say Kedusha De-sidra in Shaĥarit of Shabbat since one’s obligation to study words of the Prophets has already been fulfilled by reading the haftara. Nevertheless, to avoid canceling its recitation altogether, it became customary to recite it before Minĥa, thus adding extra Torah study on Shabbat, particularly study that pertains to God’s holiness (see Peninei Halakha: Prayer 23:2).