Literally means 'to return' and in an Islamic context, it refers to the act of leaving what Allah SWT has prohibited and returning to what He has commanded.
Asking for forgiveness for our sins is something that must be sought sincerely and with consciousness. We should remember that it is only through Allah's Mercy that anyone will ever enter Paradise.
In Surah Az-Zumar (39:53) Allah SWT says : "Say, O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah. for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful."
We should never lose hope or stop asking for Allah's forgiveness. The Prophet Muhammad, Sallallaahu 'alaihi wa sallam said: "O people! Turn to Allah in repentance and seek His forgiveness, for surely I make repentance a hundred times every day." (reported by Sahih Muslim)
The Prophet Muhammad, Sallallaahu 'alaihi wa sallam advised: "Do good deeds properly, sincerely and moderately, and rejoice, for no one's good deeds will put him in Paradise." The Companions asked, "Not even you O Messenger of Allah?" He replied. "Not even me unless Allah bestows His pardon and mercy on me." (reported by Abu Hurairah & 'Aa'ishah & collected by Al-Bukhari.)
Never despair of Allah's Mercy - no sin is too great to repent for or too little for that matter.
(excerpt from : http://islamworld.net/docs/repent.html)