Active transport is when molecules move across a cell membrane from a lower concentration to a higher concentration. This takes energy, often from adenosine triphosphate (ATP). Active transport is done so cells get what they need, such as ions, glucose and amino acids.
In general, molecules move from an area of higher concentration to an area of lower concentration. To get molecules into the cell against the concentration gradient, work must be done. The work is done in special proteins which act as ports in the cell membrane. The imports must come through the ports: they cannot get through the bilipid layer of the cell membrane.