A municipal drain is a system to move water. It is created pursuant to a bylaw passed by the local municipality. The municipality is responsible for the construction of the drainage system and future maintenance and repair. Costs may be recovered from the property owners in the watershed of the drain.
The Ontario Ministry of Agriculture, Food and Rural Affairs gives a good overview of Municipal Drains. If you are a landowner and have questions about the drainage act, you can view a fact sheet.