Trimming: The number of base pairs that will be removed from either the 3' or the 5' end. In QIIME 2, you'll most commonly see trim-left (which will trim off the requested number of base pairs from the 5' end), but in qiime feature-classifier extract-reads, there is also a trim-right parameter, which refers to the 3' end.

Truncating: The position at which reads will be truncated (counting from the 5' end). Reads that are shorter than the value provided will be discarded. The command you'll see this action represented by within QIIME 2 is trunc-len. Something to note is that in the denoising pipelines within the QIIME 2 core framework, truncation occurs before trimming.