The "StringFormat" composite formatting actions (both server and client) takes a list of objects and a composite format string as input.
A composite format string consists of fixed text intermixed with indexed placeholders, called format items. These format items correspond to the text in the list. The formatting operation yields a result string that consists of the original fixed text intermixed with the string representation of the objects in the list.

For developers familiar with C#, it is an action that attempts to replicate ( without individual item formatting functionality ) the String.Format method

The syntax for a placeholder is {[INDEX]} where [INDEX], also called a parameter specifier, is a number starting from 0 that identifies a corresponding item in the list of objects. That is, the format item whose parameter specifier is 0 formats the first object in the list. The format item whose parameter specifier is 1 formats the second object in the list, and so on.

The matching braces { and } are required.

Example of a string with placeholder:
" Hi {0}, My names is {1}"