Numbers, text, and images are all examples of data — but they’re not all the same type of data. You can do things with a number (such as calculate the square root) that wouldn’t make sense with a sentence of text. Similarly, you can do things with an image (such as applying a kaleidoscope filter) that wouldn’t make sense with a number.
In Vuo, data is categorized by data type. A node can only input and output certain data types that make sense with the job that the node does, such as calculating numbers or filtering images.