It is looking at your life and looking to see what sins you have committed and estimate how many times or how often you committed them since your last confession. Sometimes you might look further back, if there was something you had forgotten about, or just thought about and is bothering you. In order to confess, you have to think about what sins you are guilty of. It is part of the standard prep—sure you can go in there cold, but you would technically be examining your conscience while in the confessional. I myself usually do my examination beforehand, make a list (which I keep secret and try to discard discreetly asap afterwards), revise it—sometimes I add things I didn’t think of, sometimes I decide I am being scrupulous or being too detailed when it isn’t necessary for the Priest to understand the nature of the sin. You don’t need to be overly explicit in most cases as long as you get the basic idea of what kind of sin it was across.
But sometimes details do matter when evaluating the gravity of sins. For example: lying and dishonesty. Telling someone her new hairstyle looks good when in your opinion it looks silly is not a particularly serious sin. Lying under oath and saying someone committed a crime they did not is very serious. This kind of distinction would be important in confession.