What does this probability notation denote: $P( Y=y | X =x)$?

Question

What does this probability notation denote: $P( Y=y | X =x)$? I came across this while looking at my notes for optimal prediction rule.

Optimal prediction rule:
for each $x ∈ X$ , chose $y ∈ Y$ with maximum $\Pr[Y = y | X = x]$.