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The second bracha of Shemoneh Esrei is the bracha of Gevuros (Hashem's might). Chazal teach us that when Avraham placed the knife on Yitzchok's neck, at the Akeida, Yitzchok'sneshama (soul) left his body. When his neshama heard the Angel tell Avraham "do not place your hand on Yitzchok," the neshama returned to the body and Yitzchok stood up. At that moment Yitzchok understood that in the future Hashem will resurrect the dead and he proclaimed, "Baruch atah Hashem, Me'chayei Ha'meisim (Blessed are You Hashem, the One who resurrects the dead)" (Pirkei D'Rebbe Eliezer chap. 30).

The Mishneh [Berachos 33a] says that we mention the strength of [the wind and] rain during the bracha mentioning techiyas ha'meisim. At a later point in Shemoneh Esrei, in birchas ha'shanim (blessing for livelihood) we ask for rain. Rashi [ibid. s.v. maz'keerin] explains that when we recite mashiv haruach u'morid hageshem we are not actually requesting rain, rather we are mentioning that Hashem controls the rain, which is a praise of Hashem.

The Gemara [Berachos ibid] explains that rain is equivalent to the resurrection of the dead. Rashi [Taanis 7a s.v. u'pleegee] explains that rain causes produce to grow which helps sustain life. Rabbi E.E. Dessler, zt"l, offers an additional explanation. If a person would attempt to imagine how techiyas ha'meisim will look, one can only visualize unbelievable miracles. Yet each year when the trees and plants die and again grow the following Spring, no one even takes notice of it. If Hashem would have made the world that the opposite takes place, i.e. that people die and shortly after are resurrected, yet plants once they die they remain dead for hundreds of years, if one were to observe a plant growing again, he would feel as if he just observed a great miracle. It is only because we are accustomed to plants dying and growing again that we are not amazed when this miracle occurs. However, if a person thinks about it, the concept of humans and plants living after death is the exact same miracle (Sifsei Chaim, Shemoneh Esrei, pg. 42).

With regards to mentioning rain, there is a difference in text between the winter and the summer. In the winter we mention mashiv haruach u'morid hageshem (Who makes the wind blow and brings down the rain), and during the summer months we mention morid ha'tal (Who brings down the dew) (See Shulchan Aruch 114:3). Generally, b'nei Ashkenaz do not mention anything in the summer months (Rama 114:3), and b'nei Sefard say morid ha'tal (See Shulchan Aruch 114:3). In Eretz Yisroel, the minhag of even b'nei Ashkenaz is to say morid ha'tal during the summer.

We mention mashiv ha'ruach beginning at Musaf of Shemenei Atzeres until Musaf of the first day of Pesach (Shulchan Aruch 114:1). The Mishneh Berurah [114:2] explains that we should begin asking for rain at Maariv on the first night of Succos, since on Succos we are judged on rain (Mishneh Rosh Hashanah 16a). However, since rain is considered a (bad omen) on Succos (See Mishneh Succah28b & Rashi ibid. s.v. mashal) since one is unable to sit in the Succah, we push it off to immediately after Succos [i.e. Shemenei Atzeres]. Logically since the [halachic] day always starts at night, we should begin mentioning mashiv ha'ruach at Maariv of Shemenei Atzeres, however since not everyone is in shul at night, Chazal did not want to start without the whole congregation present, to avoid confusion. We cannot start by shachris, since one may not interrupt between birchas krias shema and shemoneh esrei [s'michas geulah l'tefillah (Shulchan Aruch 111:1)], and therefore no one may announce before shemoneh esrei that we are switching over. Therefore, we do not switch until Musaf. Since the proper time to switch would have been starting from Maariv, if one accidently mentioned mashiv ha'ruach before Musaf, one would not need to repeat Shemoneh Esrei (Mishneh Berurah 114:3).