Istighfaar means seeking forgiveness of Allah for the sins so far committed and turning repentant unto Allah.
Ibn Abbas relates that the was in the company of Imam Ali when a man came and said Mawla, I have transgressed much "Imam Ali told him: PRAY ISTIGHFAAR. Another man came and said : Mawla my land has become arid and parched, what shall I do ?. Imam Ali told him PRAY ISTIGHFAR. After a few moments another came and said Mawla I am poverty stricken. Nothing I do bearts fruit. Is there any remedy. Imam Ali said: PRAY ISTIGHFAR. Then another man came and said : Mawla Allah has not yet blessed me with a child. What should I do. Imam Ali told him PRAY ISTIGHAFAR.
I asked him : O AMIRUL MUMININ - though the problems were different, but to each of them you advised to pray ISTIGHFAAR. Imam Ali said : Because Allah says in surah Nuh:
And I said : seek forgiveness of your Lord. Verily he is ever most forgiving. He will send you abundant rain from the heaven and will help you with wealth and sons, and will make for you gardens and will make for your rivers.