In Taverna 2.4 and later, String Constant has been replaced by Text Constant.
A String constant service allows you to set a fixed-value input for a service or the whole workflow. This is useful for inputs that are fixed across multiple runs of the same workflow, and also for publishing your workflows since the string constant provides an example input for people wanting to try out the workflow.
Note that you can add a String constant from the Insert menu as well.
Alternatively, you can drag in a String Constant from the Service Templates section of the Service Panel, as explained in the previous section, and connect it to the appropriate input port of your service.