Sillage is the french word for the wake a passing ship leaves behind it. As far as fragrances go, to me it means the trail of fragrance or wafting scent others can smell emanating from the skin. More is not always better. A fragrance with little sillage may be described as a skin scent, a personal scent. These may last a long time on the skin but pretty much only be appreciated inside twelve inches. They are appropriate for occasions when people are close together--movies, plays, classes, business meetings. Other fragrances are 'sillage monsters' and well known for turning heads--hopefully in a good way but you never know. They can leave your presence in a room when you are already in the next building! A fragrance like this might be chosen for an outdoor concert, or perhaps for clubbing in a smoky bar.
These are my impressions of the topic, and I'm sure others will chime in!