The lay man sees it in the most casual and natural way. Slidenfil brings about a stronger and longer penis erection. To understand how Sildenafil works, you must first understand how the penis becomes erect. The easiest way to understand that is to think about a balloon. If a balloon has no air in it, it is limp. When you begin to inflate a limp balloon with just a little air, it becomes elongated and rigid.
The penis uses a similar method. Instead of using pressurized air to become rigid, the penis uses pressurized blood. Inside the penis, there is two long tubes shaped like cigars. The structures are called the corpora cavernosa and that is what it uses to become erect.
Arteries bring blood into these two tubes and the veins carry blood away from them. Depending on the flow of blood, the penis will be either limp or erect.
If the penis is not erect, the arteries that bring blood into the cigar like tubes are somewhat constricted while the veins that drain the blood are open. There is no way for blood to build into the penis; in this state the penis is limp.
When a man gets sexually excited, the arteries going to the penis open up so that the pressurized blood can enter the penis fast. The veins leaving the penis constrict, or close off. The blood then gets trapped in the cigar tubes and then the penis elongates and stiffens. That means the penis is erect.