It is a very rough representation of all the ideas.
Sketches can be done on paper with a help of a pencil, and it is actually the preferred way because of the overwhelming amount of options in digital tools that might get on your way, and you might end up deciding what is the best line/font/shape to use instead of thinking about the core ideas.
But you are free to choose some digital tools, but it is easier to create a sketch on paper.
By simply outlining the main elements and ideas you will be able to quickly show your suggestions on the structure of the future app to all stakeholders.
The details should be omitted, you will have time to focus on them later.
When all features and main elements are presented on paper and you continue developing them on a more profound level, the rough sketches turn into detailed sketches, and they now can be called wireframes.
