Starching means treating clothes with a substance to make them stiff.
Starching is a way to make clothes look nice and neat.
Examples:
She is starching her shirt.
I will be starching my dress for the party.
He is always starching his collars.
They are starching their uniforms for the parade.
The old lady is starching all her clothes to make them very stiff.
I am starching my socks to make them stand up by themselves!