calculus and analysis – What does the ‘ (prime) mean in this context?

I’m trying to check a simple calculation regarding the derivative of a matrix exponential. By hand I get
$$frac{d}{dtheta_1}e^{theta_1 mathbf{W_1}+theta_2mathbf{W_2}} = mathbf{W_1}e^{theta_1 mathbf{W_1}+theta_2mathbf{W_2}}.

Checking this with Mathematica, I have

Assuming ({(Theta)1 (Element) Reals, {W1, W2} (Element) 
   Matrices({d, d}, Reals, Symmetric)}, 
 D(MatrixExp( ((Theta)1 W1 + (Theta)2 W2)), (Theta)1))

which returns

(* W1 Derivative(1)(MatrixExp)(W1 (Theta)1 + W2 (Theta)2) *)

Why is there a derivatie here? And what’s the (1)?