Hosanna is an English exclamation of praise or applause, that may be frequently found in Christian liturgy. Its rarely heard these days now that Christianity in Britain is a minority pursuit roughly as popular as angling.
Its origin is Hebrew. Hosanna is a contraction of hoshiah-na, which means Save, [we] pray (see Psalms 118:25).
On every day of Succot, during the morning service, the ceremony of Hoshanot takes place. The community encircles the bimah carrying their the Torah scrolls and their four species (palm branch, willow, myrtle and citron) and recite a series of prayers repeating the refrain, Hoshana, Save, please.
This recreates a ritual that took place in the Temple. The Talmud describes how on each day of the festival, people would walk around the altar carrying their four species and singing Hoshiah-nah (Succah 45a). The seventh day of Succot is called Hoshana Rabba, literally, the Big Hoshana, because on that day we encircle the bimah not once but seven times. The bundle of willow leaves that we carry on Hoshana Rabba is sometimes called hoshanot in honour of their role in the ceremony.
The idea behind the custom is that Succot is the time when the rainfall for the coming year is allocated (Talmud, Rosh Hashanah, 16a). Hoshanot are a plea that we may be judged generously for rain, and for material blessings to be more widely understood in the coming year.