I'm going to assume you're talking about the backside of the desktop computer? If so, you'd have to take the cover off of it to expose the guts. There is a big flat circuit board inside. This is called the "motherboard" or "mainboard".
If you look at the expansion slot from the inside you'll see the same metal plate covering it. This is to prevent dust from entering the system, and enhance cooling by controlling airflow. The plate can be removed from the inside, but I don't recommend it unless you're adding an expansion card to that slot.
Now, if you'll look at the motherboard right by that metal plate, you'll see a "slot". There are many different styles, based on the age of your computer, but in any case, it is where you might plug in an expansion card of some kind. Nowadays, just about every new machine has everything built into the motherboard, so there's not a lot of expansion to be done. But you may want to add a better graphics card, a second Network Interface Card, or something. That's what the slot is for. If I'm understanding your question, that is.