The bit where it says transform.position selects just the first component of the array (just to be annoying, most computer languages count array components from 0, so the first component is, the second and so on). Note that the enclosing square brackets are part of the expression - they turn the three values back into an array. Say the layer's Y position is 123 and its Z position is 345.
We'll use the X component of the transform.position array and then put in a constant value for the Y and Z components. So say we want to lock the layer to movement only on the X axis.
So we have to pick the array apart into its components, and then create a new array using some constant values. We want to lock it so that only one of the values can change. The position property is an array – it contains the X, Y and Z value components. You'll see the default expression transform.position is automatically inserted. Alt/Option-click the stop watch for the position property.
