A test fixture is the prepared setup or sample information a test needs before it can run. It is like setting out the same ingredients and tools each time you try a recipe.
For example, a test may need a pretend user account and a few sample messages. Preparing them in the same way makes the result easier to repeat and understand.
Fixtures should be kept small and relevant to the behavior under test. If a test depends on changing real data or an outside service, it may become difficult to run reliably.