when speaking of cutting the bass for instance, below 250hz, what does this mean? Is this referring to decreasing or eliminating the frequency of this instrument or simply turning down the decibal output?
No no no!!
The decibel output is the volume. The "X frequency" is how high or low the note is.
The difference is that cutting 75 Hz (for example) lowers the level of only a tiny slice of the frequency spectrum. Lowering the decibel level of that track will turn down ALL the freqs it has, not just a thin slice as before.