So, imagine you had some substance in a bowl. The bowl has a little wall going through the middle, with no openings or anything. On one side of the wall, there are 100 molecules of the supstance. On the other, there are 10 molecules. Now, imagine that you make an opening in that little wall that separates the substance into two parts. Normally, the 100 molecules would start pouring into the part with 10 molecules, or “go down the concentration gradient”, which means going from the part with higher concentration into lower until they are equal. Going /against/ the concentration gradient is just the opposite — if the 10 molecules went into the side with 100, which takes energy to do.
