Short version: Fiji Is Just ImageJ
Long version: Fiji is an extension of the ImageJ core/base and it comes bundled with a whole bunch of useful plugins and goodies. It's a really good place to start with ImageJ.
ImageJ/Fiji Hyperstacks let you to view multi-dimensional images:
All the usaul suspect and more:
Relating image units to physical units i.e., what size does each pixel represent in metric units. Depends on the optical path that projects photons onto the detector or sensor.
Calibration are usually encoded in the image metadata in modern microscopes, but images for older microscopes need to be calibrated.