I am particularly perplexed by the following footnote. Can someone explain to me what is meant by this? > The basic concept of symbolic languages comes directl

I am particularly perplexed by the following footnote. Can someone explain to me what is meant by this? > The basic concept of symbolic languages comes directly from work in mathematical logic stretching back to the 1930s and before, but other than in the Wolfram Language it’s been very rare for it to be implemented in practice.