For example, I have a function
[power, capacity] = function_name(users, distance, radius)
and the function call is
[~, capacity] = function_name(users, distance, 5);
~ just means that you don't want to store the result in any variable.
here is a detailed explanation: http://www.mathworks.com/matlabcentral/answers/72537-what-does-a-tilde-inside-square-brackets-mean
And the most relevant section: "when you use [~,palette], that means that you just want the second output of your function, and do not care the first one."