One custom is to blow the shofar, the ram’s horn, each morning (except Shabbat) at the conclusion of the Shacharit (morning) prayer service. Some say this is to help us prepare ourselves and “wake us up” as we get ready for the new year. The shofar’s call is a harsh and unusual sound.
The shofar is only blown from the 2nd to the 28th day of Elul. It is not blown on the day immediately before Rosh HaShanah, so as to make a careful and clear distinction between the mitzvah (commandment) of blowing the shofar during Elul, and hearing the shofar on Rosh HaShanah.
On a purely practical level, it helps the Ba’al Tekiyah (person who blows the shofar) prepare physically for the difficult task of performing the full ritual of the Shofar Service on Rosh haShanah. Blowing the shofar takes practice and skill, as well as lung strength!