The developer console is, in short terms, a text box that you can type commands into. These commands can do all sorts of things like change your FOV, change your graphics settings, interact with the server, set up binds.. The list goes on!
Unfortunately, the developer console isn’t enabled by default. It’s not too hard to enable it though, all you have to do go to your settings menu, click “Game Settings”, and select “Yes” under the “Enable Developer Console” option. Here is the path in an easier-to-read format:
Settings > Game Settings > Enable Developer Console > Yes
After enabling the developer console, you can open it from anywhere in CS:GO by hitting the ~ key (tilde) on your keyboard. For some, the ` key (grave) will also open the console.
Below is an image of the settings screen with the buttons to select highlighted in red: