It indicates that it is an anonymous function. Sort of like a short one-liner macro type of function rather than a full blown function like you're probably used to. Maybe they call it anonymous because no one understands them or knows what they are or how to use them, as is evident in your case, and with many other novices. :-)
If you like, you can use a traditional function instead of that cryptic one-liner form.
function output = SNR(signal,noisy)
output = 20*log10(norm(signal)/norm(signal-noisy));
The @ in the line indicates that it's now going to list the input arguments. They are listed separated by commas and wrapped in parentheses.
