x_1 is just a number...
so basically you plug numbers into those weird looking symbols
eg if x_1=2, x_2=5, y_1=3, y_2=7
then cauchy-schwarz inequality says that
(2^2+5^2)*(3^2+7^2)>=(2*3+5*7)^2.
Which if you expand out you can see is true.
PS x^2 means x squared...