Codification, in this context, means that blocks are converted to a string. Many people use Codification Support blocks made by Snap! to encode blocks, in most cases, to a programming language or something similar!
Reverse Codification is the counterpart of Codification. It uses a string to convert it as a block. Since Snap! primitively doesn't provide any block, users have to create their own blocks to do this task. These are used, for example, to store blocks in browser!