what does "against the concentration gradient " mean???????

 suppose a cell has an 1M solution of NaCl in it. outside the cell there's a 2M solution of NaCl. there's a difference in concentration between those two places, and NaCl "wants" to flow from regions of higher concentration (outside the cell) to regions of lower concentration (inside the cell). that's the "concentration gradient"; the difference in concentration, plus the direction stuff wants to flow.

in order to move NaCl "against" the concentration gradient, i.e., to move NaCl from inside the cell to outside the cell, some kind of work has to be performed (for example, via active transport). diffusion alone won't cut it. (that's a result of the second law of thermodynamics.)

in real cells, of course, the concentration gradient isn't the only kind of gradient thats important.