That's not a function, and it's not valid MATLAB syntax either. It's hard to say what it means, because I have to assume that it's assembled from unrelated pieces.
This
t=0:1/fs
creates a simple vector from 0 to 1/fs with a step size of 1. See colon, :
This
length(x)-1
returns the scalar length of the array x, minus 1. The "length" of an array is not dependent on its orientation; rather, it's the size of the longest dimension.
This
t=0:1/fs [length (x)-1]
will simply return an error. Implicit multiplication isn't allowed in MATLAB, so the juxtaposed expressions will result in an error. While not an error, wrapping the second expression in square brackets accomplishes nothing. Square brackets are used for concatenation, so you're trying to concatenate a scalar.
