Fixture "get_tf" called directly. Fixtures are not meant to be called directly

When running py.test under linux I get this error message. I'm not sure what fixture are.

platform linux -- Python 3.6.5, pytest-4.1.1, py-1.5.3, pluggy-0.8.1
rootdir: /topfarm, inifile: setup.cfg
plugins: requests-mock-1.5.2, remotedata-0.2.1, openfiles-0.3.0, doctestplus-0.1.3, cov-2.6.1, arraydiff-0.2
collected 221 items / 2 errors
===========================================
_________________________________________________ ERROR collecting topfarm/tests/test_main.py _________________________________________________
Fixture "get_tf" called directly. Fixtures are not meant to be called directly,
but are created automatically when test functions request them as parameters.
See https://docs.pytest.org/en/latest/fixture.html for more information about fixtures, and
https://docs.pytest.org/en/latest/deprecations.html#calling-fixtures-directly about how to update your code.
________________________________ ERROR collecting topfarm/tests/test_constraint/test_constrained_generator.py _________________________________
Fixture "get_tf" called directly. Fixtures are not meant to be called directly,
but are created automatically when test functions request them as parameters.
See https://docs.pytest.org/en/latest/fixture.html for more information about fixtures, and
https://docs.pytest.org/en/latest/deprecations.html#calling-fixtures-directly about how to update your code.