It's an anonymous function handle. Which represents single inline executable expressions that return one output.
To define an anonymous function, enclose input argument names in parentheses immediately after the @ operator, and then specify the executable expression.
For example, create a handle to an anonymous function that evaluates the expression x2 − y2:
f = @(x,y) (x.^2 - y.^2);
Anonymous functions can accept multiple inputs but return only one output.
In your question the input argument names are : XTRAIN, ytrain,XTEST
While, the executeable expression is : classify(XTEST,XTRAIN,... ytrain)
