C# Single slash and double slash

Question

Hello, what is different between single slash directory and double slash directory. 

Sometimes it could be

C:\\Users\\Mammamia\\Videos\\Documents\\CFD\\geo_msh\\cubeOp.txt

OR

C:\Users\Mammamia\Videos\Documents\CFD\geo_msh\cubeOp.txt
When we want to use this and why you want to do that ?