What does this mean A Level Mechanics

I've attached the diagram the book drew below but I don't understand how to apply that sentence onto it

The questions states that "the line of action of the force is in the same vertical plane as the line of the greatest slope"?

It is really saying the problem exists in a 2D plane, i.e. the force F is not pushing the particle in/out of the paper. Although this is generally implied in such problems, it's a technicality they're pointing out.

When you walk up a hill, you might walk in a zigzag because it is less steep than walking straight to the top. The straight line directly to the top is the line of greatest slope.

The question is saying there is zero component of the force which is perpendicular to this aspect needing to be resolved and so the motion is in line with the greatest slope (as opposed to coming down at a slant).

Wait... but the motion isn't in line with the slope? It's like, head on, isn't it?

Yes the Force and motion is head on, in a line that goes from the top of the plane to the bottom of the plane.

This line is "the line of greatest slope".

The plane only looks like the picture from side-on. It is more like a rectangular table surface tilted at an angle.

You can imagine if you look at the tilted table from side-on and a ball rolls from the top to the bottom you couldn't tell if it was rolling straight down the middle from (midpoint of top to midpoint of base) (the line of greatest slope) or from top right to bottom left (a slanted path).

The question is saying the force acts like the former case, none of this slanty business.

Oh that MAJORLY clears it up... in retrospect I don't see how I didn't get that! I guess I forgot to consider what it looked like out of 2D. Thanks so much!