What does fMRI data look like after pre-processing?
The MRI signal is a small electrical current induced in the receiver coil by the precession of magnetization during resonance, i.e., a manifestation of Faraday's Law of Induction, wherein a changing magnetic field induces a voltage in a nearby conductor. The BOLD response often used in fMRI measures the hemodynamic response, i.e., the relative levels of oxyhemoglobin and deoxyhemoglobin that can be detected on the basis of their differential magnetic susceptibility.

As any digital signal, there are limits to the resolution of fMRI, and is dependent on a number of factors (Kashou, 2014), including the field of view (x- and y- directions) and the slice thickness (z-direction). Multiplying the x, y and z sizes results in the total voxel size (Fig. 1). each voxel has 1 value, namely its respective BOLD response signal value. 

Reference
- Kashou, A Practical Guide to an fMRI Experiment, Intech Open Science (2013)


Fig. 1. Time series of fMRI. source: Principles of fMRI