Tests need to run against the background of a known set of objects. This set of objects is called a test fixture. When you are writing tests you will often find that you spend more time writing the code to set up the fixture than you do in actually testing values.
The purpose of a test fixture is to ensure that there is a well known and fixed environment in which tests are run so that results are repeatable. Some people call this the test context.
Examples of fixtures:
Loading a database with a specific, known set of data
Erasing a hard disk and installing a known clean operating system installation
Copying a specific known set of files
Preparation of input data and set-up/creation of fake or mock objects
One of the most time-consuming parts of writing tests is writing the code to set the world up in a known state and then return it to its original state when the test is complete. This known state is called the fixture of the test.