When you create a site space, Confluence will suggest a space key or you can enter your own key to make it more memorable.
For example, you might give your marketing team's space a key of MARKETING. You can then navigate directly to the space using a URL like this -
Personal spaces always use your username as the space key.
Choosing a space key
Each space key:
Must be unique.
Can contain any alphanumeric character (a-z, 0-9).
Can be up to 255 characters long.
You can't change the space key after you create your space, so choose your space key carefully!