Isteqahara is a way of seeking guidance from Allah. Sometimes we come at crossroads in life when we have more than one option before us but no idea about which path would suit us the best. It is at such times that we turn towards Allah so that he may show us the right way.
A person performing an Isteqhara makes a humble request to Allah to tell him if what he intends to do would be beneficial for him in the larger picture or not. Then the person obediently follows the way that he is shown by Allah.