In samatha you do a thing with your attention.
Your attention is basically your root sense. The sense behind your seeing, thinking, hearing, smelling etc.
Your attention is usually all boggled and agitated, so it doesn't see so good.
In samatha you de-agitate your attention. Make it calm. When it's calm you see better. Much better. Stuff that was invisible becomes visible.
Samatha is a refined form of concentration. All of the people here are pretty familiar with concentration and what it's good for. So in samatha you just take it further.