What is the no-slip condition? What causes it?

The no-slip condition for viscous fluids assumes that at a solid boundary, the fluid will have zero velocity relative to the boundary. Along with a flow when adhesion is stronger than cohesion ... Basically the molecules of the fluid crash into the molecules of the wall and get stopped.

The no-slip condition is the viscosity of a fluid. This is most likely brought on by stress or stressful situations.