Enhance the router signal

Hello, I need a Wi-Fi Range Extender, that I take the signal the powers a little.

The technology of these accessories and to take a signal and to strengthen it as a router or a switch -> Range Extender Wi-Fi -> and the client has any signal attenuation?
It remains the same as that which bears the router?
It strengthens this so that you have the strongest signal for clients.

My problem.
Standar router Telecom in a third-floor apartment of a building where the ground floor there is the local (shop with three slides) which must pick up the signal. Now only with the router the signal arrives but, weak, and there is a local point where the signal and stronger, enough to be able to navigate with a non-performing smartphone.

We would like to put a Range Extender Wi-Fi in the strongest step to bring it to the client.

Your Wi-Fi Range Extender are all uquali for the work that I do, or I have to evaluate some of the same characteristics.

I explained enhancement, signal loss?

Thank you.