A flagellum is a lash-like movable appendage that protrudes from the cell body of many bacteria and some eukaryotic cells. Including notable examples in plants (e.g. fern spores) and even mammals (e.g. sperm cells). The primary function of a flagellum is for movement, but its method of operation is yet to be explained.
“Many species of bacteria swim to find food or to avoid toxins. Swimming motility depends on helical flagella that act as propellers. Each flagellum is driven by a rotary molecular engine–the bacterial flagellar motor–which draws its energy from an ion flux entering the cell. Despite much progress, the detailed mechanisms underlying the motor's extraordinary power output, as well as its near 100% efficiency, have yet to be understood.”
Source: Steps in the Bacterial Flagellar Motor PLOS Computational Biology