Psalm 92, known as Mizmor Shir L'yom HaShabbat, is a psalm the was written for the Shabbat day. Though it is permissible to recite it any day, it is generally reserved for Shabbos and is also recited during the morning services on festivals.
During Shabbos, it is recited twice, once during Kabbalat Shabbat, and once during morning services. The recitation during Kabbalat Shabbat officially ushers in the Shabbos.
Psalm 92 has a deep meaning: During the week, we are all so busy, we do not get to spend a lot of time in service to G-d. But on Shabbos, our day of rest, since we put all our weekday worries aside, we have more time for G-d.
The information contained here is referenced in the Artscroll Book of Psalms in the commentary of Psalm 92.