When we talk about a "language of reading," we refer to a set of rules, patterns, codes and conventions that people interpret in order to make sense of a given text. Readers with a lot of experience and a developed "language of reading" are usually more skilled at recognizing patterns and conventions within a text, such as themes, characters and main ideas.
Memory, symbols and patterns all contribute to the development of a "language of reading." Memory allows us to remember past readings and to apply these experiences to new readings that we encounter. Symbols and patterns allow us to interpret the various meanings of a text and connect them to other texts we have read before. In this way, we are able to develop our "symbolic imagination."
"Symbolic imagination" refers to an ability that humans have of thinking about a subject beyond its immediate meaning. This way of thinking allows people to understand ideas that are not explicitly stated but that are suggested by a text. By having a developed "language of reading," we are able to connect the ideas of a particular text with those of texts we have read in the past, thus developing our "symbolic imagination."