According to the documentation for Function Inputs and Function Outputs, the tilde simply means "ignore" input or output. It is helpful when you want the first and third outputs, but not the second, from a function. Using tilde tells MATLAB not to waste memory in your workspace by keeping that variable.
In your specific case, xlsread returns 3 outputs: the numbers, the non-digit text, and the raw information (numbers and text combined in one cell array). The author of your example line of code is saying "I don't care about the text output variable." Either because it will not be used at all, or because it will be extracted from the raw.
