A short is a 16-bit data-type.
On all Arduinos (ATMega and ARM based) a short stores a 16-bit (2-byte) value. This yields a range of -32,768 to 32,767 (minimum value of -2^15 and a maximum value of (2^15) - 1).
short var = val;
var: variable name.
val: the value you assign to that variable.
short ledPin = 13