How are mocks different from Fixtures

I’m writing tests for a rails app. Its a legacy app and no tests were
written to cover the code. In the test code, I’m hitting the test
database often and it makes the tests run very very slow.

What are mocks and what are all the stuff I can mock? How are mocks
different from fixtures? Been reading some articles about this but I
really can’t understand fully the significance of it in my test code.