Difference between going against and going down a concentration gradient
"Concentration" is "how much stuff is there someplace?" "Concentration gradient" is "how much is concentration changing from point A to point B?"

Imagine a terrain where concentration is represented by height.

"Down" a concentration gradient means from high concentration to low concentration - this is the direction things would move via simple diffusion.

"Up" or "against" a concentration gradient means from low concentration to high concentration. Moving in this direction requires some sort of energy input, such as active transport using ATP, or an electrical gradient, or something else.