It's often used as part of place names, indicating that the place is located near or on a lake. For example:
* Château du Lac: Castle of the Lake
* Lac du Bourget: Lake of the Bourget
* Parc du Lac: Park of the Lake
It can also be used in more general contexts to describe something related to a lake, such as:
* Cuisine du lac: Cuisine of the lake (referring to food typical of areas near lakes)
* Vue du lac: View of the lake
The specific meaning of "du lac" will depend on the context in which it is used.