What gradient actually means
Students who have only met m as a letter in y = mx + c have usually never seen what it measures.
Most students can recite that m is the gradient. Far fewer can say what it is the gradient of, or why a steeper line has a bigger one. That gap shows up later as guesswork on every straight-line question.
Drag it and watch
Drag either dot. Make the line steeper — what happens to the rise? Now make it go downhill. What changes about the number?
Two things trip students up. A line going downhill has a negative gradient, because the rise is negative. And a vertical line has no gradient at all — the run is zero, and you cannot divide by zero.
Once gradient is a triangle rather than a letter, parallel lines, perpendicular lines and rates of change all stop being separate rules to memorise.