Hi-Fi: High fidelity. Close to the original recording. A neutral or linear presenation that is free from (most) colorations. Nearly a mirror image of the music, but not exactly. You can hear fine details with ease and pick them apart easily.
Mid-Fi: Mid-fidelity: Like Hi-Fi but with more pronouced colorations such as treble sparkle, midrange forwardness, and low-bass rumble etc. Rather tasteful colorations that can be easier on the ears; while still providing adequate detail retrieval.
Low-Fi: Low fidelity: An abundance of colorations that detract from the efforts of live sound or well recorded music. Detail retrieval is not found here. Overall, a poor replication of the original that cannot compete with the above at all.