The term "string grammar" in computational linguistics refers to the structure of a specific language, such that it can be formatted as a single continuous string of text, without the need to have line-breaks (or newlines) to alter the meaning. The appearance of any text in "column 1" (or any column) of a line does not change the meaning of that text in a string grammar. A string grammar can be used to describe the structure of some natural languages, such as English or French, as well as for some computer languages.
Note that the string-based structure is for defining the grammar of a language, rather than the formatting of the language itself. The production rules, of the grammar, are in the form of continuous text strings.