Y intercept is the value of a function f(x) at x = 0. In the analogy, it's the starting position. Someone starting at a higher baseline knowledge is not necessarily destined to stay ahead of a fast learner who happens to start at a lower baseline of knowledge.

A number have people have replied and I think they are all correct, but I want to be explicit.

Given a function y = mx+b. Graphically, the function is a line on the xy plane, and if you trace your finger along the line toward the y axis, where your finger “intercepts” the y axis is the value of the y intercept.

That’s the idea of the name.

And everyone else is also correct, Its value is f(0) where y = f(x) = mx + b.

Y value at x=0. Also sometimes known as "initial conditions"

Y intercept is where the line crossed the y axis at time 0. So it's the initial state.

Y = mx + b