CE String is an ExpressionEngine® plugin. This means that it does not have a user interface, but is intended as a development tool to be used in your templates. It allows you, as the developer, to dynamically manipulate all-kinds of text in all-kinds of ways, on-the-fly, in your templates.
CE String takes the parameters you provide to it, and manipulates the given text in the way you ask it to. Please take a look at Basic Usage to see examples.
Simply put, CE String is the most robust and complete text manipulation solution for ExpressionEngine®. The syntax is simple. It’s powerful and the methods it gives you access to are pretty comprehensive. You can generally do more with less code.