The F pilus is also known as a conjugation bridge. It's a long, narrow structure which protrudes from the surface of the "male" bacterium and connects the "male" to the "female" bacterium during the process of bacterial conjugation, which occurs when genetic material in the form of the F plasmid is swapped between two bacterial cells.

In other words, it's roughly the microbial equivalent of a penis.

From the science dictionary at