Question

How use of detergent help us in remain dirt & grease from clothes?

Answer 1

Soap reduces the surface tension of water, making it easier for water to dissolve stuff.
Water has $"surface$ $tension"$ a tendency for water molecules on the surface of water to cling together. This creates a film that resists penetration. Soap molecules disrupt the adhesion of water molecules to each other, and so lessens the strength of that film. The water therefore comes in closer contact with other substances, which accelerates the process by which those substances dissolve into water. Water really is an excellent solvent; it dissolves lots and lots of things. Soap merely enhances this process.