There is an excellent guide to MRI at mriquestions.com. Most images in these notes come from that site, courtesy of Allen D. Elster, MRIquestions.com.

A signal can be represented as a sum of its sinusoidal components. Here the signal, $f$, is red and the sinusoidal components are blue:

[1D FT animation from wikipedia.org]

The magnitudes and phase (phase is not shown above) of the sinusoidal components form a "frequency space" representation of the same signal. This "frequency space" representation is shown as $\hat{f}$ above.

In 2D, the sinusoids are sinusoidal waves. The sum of all these sinusoidal waves at different frequencies results in an image:

The Fourier Transform converts from one representation to the other, and back:

For those who want more discussion of the Fourier Transform, there are notes and videos available in the CISC/CMPE 457 Image Processing course.

Please look at the Frequency Domain section on the 457 Schedule & Notes page.