Answer:
Moving against the concentration gradient, or in simpler terms, moving from a low-concentration area to a high-concentration area.
Step-by-step explanation:
This action uses energy, as according to entropy, the substance will move from a higher concentration to a lower concentration, and cells have to put extra energy to pump the molecules / substance up the concentration gradient.
It is mostly used to trigger chemical responses in cells, like in neuron synapses.
Hope this helped!