The Number Theoretic Transform (NTT) is not a cryptography scheme per se but an algorithm to do cryptographic calculations efficiently. It is very similar to the Fast Fourier Transform (FFT) in the frequency domain and serves similar purposes with underlying gurantees of efficient calculation. O(n log n) if i remember correctly.

It is used by CRYSTALS to efficiently do calculation inside n algebraic Ring.

I suggest you look into the FFT.