Meta-language and semantic concepts
The meta-language is a language that is introduced to describe another language. In the language of the lowest object level, semantic concepts of "true" and "false" cannot be formulated - for more details on the limitations imposed in general, read A. Tarski's "Semantic Theory of Truth...". Here, we are interested in the adjacent idea that even our natural language, which we use for conversations, in order to discuss the truthfulness of statements, must be divided into at least two languages: the object level language and the meta-language, in which various important statements about the object level language are formulated.
Next, there can be a chain of meta-languages of any length, each of which describes the previous one. The relationship "meta-language - object level language" is still maintained there, and we can talk about languages being in such relationships to each other.
For example, let's return to the chain that I mentioned at the beginning of the chapter. We have a map, and there is a legend of the map. The description and the legend, instructions on how to read the description. The map describes some world, the legend of the map describes the map itself. It turns out: object - description of the object in a certain language - description of this language (also in a certain language) - description of this language - and so on infinitely.