I just joined Air Table - why is my dashboard full of someone else's content?

Airtable loads new users with example/template bases so you can see what’s possible. You can delete a base you don’t need by clicking the down arrow next to its name at the top, or a whole Workspace at a time (similarly by clicking the down arrow next to its name).

If you intended to use the base you’ve shown as a template, you can simple delete the records that come in it by selecting them all (Crtl + A) and right clicking to delete.