W-OTS+ one-wayness property
Inside the probability brackets, everything on one side of the colon describes the distributions of the variables used to define an event on the other side. In the case you described, the success probability of $A$ is the probability that $y=f_k(x')$ (which is the event) happens for $k,x',y$ sampled/generated from the left-side operations, i.e., sampling a random key $k$ and a random $x$, then computing the output $y=f_k(x)$, and finally giving $k,y$ to the adversary $A$ and getting its output. In English, that means the adversary $A$ wins if and only if it can find a preimage of $f_k(x)$ for random $k,x$, where $k$ is known to $A$.